We earned $1.54 per diluted share versus $1.35 in the year-ago period on revenues of $2.3 billion with operating income margins of 5.1%.
We had strong revenue growth in Mechanical Construction segment, up 8.4%.
We had strong growth in our U.S. Building Services segment, up 10.3% and had strong growth in our U.K. Building Services segment, up 12.8%.
And as expected, we had a significant decline in revenues of over 35.3% in our Industrial Services segment, which was impacted not only by industry conditions but also the Texas Freeze, which in many cases, pushed out our turnaround schedule into the second quarter of 2021.
We also had a TRIR or a recordable incident rate of under one at 0.92, which was exceptional performance and again, shows our focus on safety and well-being throughout the pandemic, but really that's everyday at EMCOR because it's one of our core values.
At 8.8% in our Electrical Construction segment and 7.2% in our Mechanical construction segment, these operating income margins show that we are earning very good conversion on the work that we win, and we are executing well on our contracts, which are largely fixed-price contracts.
Our U.S. Building Services team had an exceptional quarter, earning 5% operating income margins on 10.3% revenue growth.
At 7.4% operating income margins and revenue growth of 4.5% without the impact of foreign exchange, we are doing very well.
We leave the quarter with increased remaining performance obligations or RPOs at $4.77 billion, up from $4.59 billion at year-end 2020, an increase from the year ago level of $4.42 billion.
Consolidated revenues of $2.3 billion are up a modest $4.2 million or 20 basis points over quarter one 2020.
Our first quarter results include $29.1 million of revenues attributable to businesses acquired pertaining to the period of time that such businesses were not owned by EMCOR in last year's first quarter.
Excluding the impact of businesses acquired, first quarter consolidated revenues declined $24.8 million or 1.1% organically.
United States Electrical Construction quarter one revenues of $456.2 million decreased $5.6 million or 1.2% from 2020's comparable quarter.
Excluding acquisition revenues of $6.5 million, this segment's revenues declined 2.6% organically as revenue reductions within the manufacturing and transportation market sectors were only partially offset by increased project activities within the commercial and institutional market sectors.
United States Mechanical Construction revenues of $903.9 million increased $69.8 million or 8.4% from quarter one of 2020.
EMCOR's total domestic construction business first quarter revenues of $1.36 billion increased $64.2 million or 5% and reflects a strong start to the year.
United States Building Services record quarterly revenues of $581.8 million increased $54.2 million or 10.3%.
Excluding acquisition revenue contribution of $22.6 million, this segment's revenues increased to $31.6 million or 6% organically.
United States Industrial Services revenues of $235.4 million decreased $128.5 million or 35.3% as this segment continues to be impacted by the negative macroeconomic conditions and uncertainty within the markets in which it operates.
United Kingdom Building Services segment revenues of $126.7 million increased $14.3 million or 12.8% due to growth in project activities across the portfolio as customers began to release projects which were previously on hold due to the COVID -- due to COVID-19.
This segment's results additionally benefited by $9.5 million as a result of the strengthening of the pound sterling, given the lifting of uncertainty around the terms of the United Kingdom's trade deal with the European Union that became effective on January 1, 2021.
Selling, general and administrative expenses of $224.1 million represent 9.7% of first quarter revenues and reflect a decrease of $2.9 million from 2020.
SGandA for the first quarter includes approximately $2.4 million of incremental expenses from businesses acquired inclusive of intangible asset amortization expense, resulting in an organic quarter-over-quarter decline in SGandA of $5.4 million.
Reported operating income for the quarter of $117 million compares to $106 million in 2020's first quarter and represents an increase of $11 million or 10.4%.
Operating margin of 5.1% has expanded by 50 basis points from the prior year's 4.6% operating margin.
Our United States Electrical Construction segment's operating income of $40.3 million is consistent with 2020's quarter one performance.
Reported operating margin of 8.8% represents a 10 basis point improvement over last year's first quarter as a result of a modest increase in this segment's gross profit margin.
First quarter operating income of our U.S. Mechanical Construction segment of $65 million increased nearly $20 million from the comparable 2020 period, and operating margin of 7.2% represents a 180 basis point expansion year-over-year.
Our total U.S. construction business is reporting $105.2 million of operating income and a 7.7% operating margin.
This performance has improved quarter-over-quarter by $19.7 million or 23.1%.
Operating income for our U.S. Building Services segment of $29.3 million is an $8.1 million increase from last year's first quarter, while operating margin of 5% represents a 100 basis point improvement.
Our U.S. Industrial Services segment operating loss of $2.4 million represents a decline of $17.9 million when compared to operating income of $15.4 million in last year's first quarter.
On a positive note, this segment was able to partially offset these negative headwinds with a nearly 21% reduction in first quarter selling, general and administrative expenses due to certain cost savings initiatives enacted in calendar year 2020.
U.K. Building Services operating income of $9.4 million or 7.4% of revenues represents an improvement of $3.6 million and 230 basis points of operating margin expansion over 2020's first quarter.
Additionally, operating income for the quarter benefited from approximately $800,000 of favorable foreign exchange rate movement.
Quarter one gross profit of $341.1 million represents 14.8% of revenues, which has improved from the comparable 2020 quarter by $8 million and 30 basis points of gross margin.
Diluted earnings per common share in the first quarter is $1.54 as compared to $1.35 per diluted share for the prior year period.
This $0.19 or 14.1% improvement establishes a new quarter one record for the company and also ties the all-time quarterly diluted earnings per share record, which we previously achieved in quarter four of 2019.
Additionally, we repurchased $13 million of our common stock pursuant to our share repurchase program and utilized nearly $32 million of cash and investing activities, including $24 million to fund the two acquisitions that we completed during the first quarter of this year.
Net identifiable intangible assets have decreased as a result of approximately $15 million of intangible asset amortization expense, partially offset by the impact of additional intangible assets recognized in connection with the previously referenced 2021 acquisitions.
As a result of our consistent outstanding borrowings and the growth in stockholders' equity due to our net income for the quarter, EMCOR's debt-to-capitalization ratio has reduced to 11.5%.
I'm going to be on page 11, remaining performance obligations by segment and market sector.
As I mentioned earlier, total remaining performance obligations or RPOs at the end of the first quarter were just under $4.8 billion, up $351 million or 7.9% when compared to the year-ago level of $4.4 billion.
And RPOs increased $181 million for the first three months of the year from the year-end level of $4.6 billion.
Our domestic construction segments experienced strong project growth in the quarter, with the RPOs increasing $219 million or 6.1% since the year-ago period of March 31, 2020.
All but $15 million of that is organic growth.
The $15 million belongs to a Chicago-based electrical contractor that really focuses on infrastructure that joined us in February.
Building Services segment RPOs increased in the quarter $121 million or 22% from the year ago quarter, a portion of which was the August 2020 acquisition of a Washington D.C. full-service mechanical contractor.
However, more representative of what we are now experiencing in this segment, RPOs grew $60 million or up 10% from December 31.
Those -- that commercial segment, which also includes the retrofit activity and new build, is 44% of total RPOs.
For the year-over-year and sequential quarter-over-quarter comparison, commercial RPOs increased $314 million and $216 million, respectively.
It jumped over 50, which is expansion territory in February and was over 55 in March.
It's up low double digits at 11% from a year-ago period, pretty much right before the full impact of the pandemic.
I'm now going to jump to page 12, and I'm going to give you a little updated commentary on these resilient sectors that we talk about.
We're in 60%, 70% of those areas now either electrically or mechanically.
That's really what it is in 20 megawatts or less.
I'm going to finish now on page 13 and 14.
In that initial guidance, we gave you about eight -- seven, eight weeks ago, we expected to earn $6.20 to $6.70 in earnings per diluted share.
And if you look at that midpoint, that would be another record year, Mark, after how many, 7?
And we expected to do that on $9.2 billion to $9.4 billion in revenue.
And so with that, we're going to raise the low end of our guidance range to $6.35".
That's a $0.15 movement from the $6.20.
And we're going to take the top end of the range up about a $0.05 or $6.75 per diluted share.
We're doing this across 4,000 projects of size of $250,000 or more.
But if you added up all our projects, we're doing this now over about 12,000 projects and service events.
We talked about that we were operating near 100% capability.
We told you we expected to have organic reduction of around $15 million, $20 million.
If this larger infrastructure package of about 50% of the money, give or take, is stuff we could participate in or projects we could potentially participate in, that for the most part is a late '22, early '23, likely late '23, early '24 event for us.
And I think let's all recall late 2008, early 2009.

Summary:
We earned $1.54 per diluted share versus $1.35 in the year-ago period on revenues of $2.3 billion with operating income margins of 5.1%.
Diluted earnings per common share in the first quarter is $1.54 as compared to $1.35 per diluted share for the prior year period.
And we expected to do that on $9.2 billion to $9.4 billion in revenue.
And so with that, we're going to raise the low end of our guidance range to $6.35".