First lieutenant First lieutenant is a commissioned officer military rank in many armed forces and , in some forces , an appointment . The rank of lieutenant has different meanings in different military formations ( see comparative military ranks ) , but the majority of cases it is common for it to be sub - divided into a senior ( first lieutenant ) and junior ( second lieutenant ) rank . In navies it may relate to a particular post rather than a rank . Israel . Israel Defense Forces . In the Israel Defense Forces , the rank above second lieutenant is simply lieutenant . The rank of ( קצין מקצועי אקדמאי ( קמ " א ( " katsín miktsoí akademai " or " kama " ) , a professional academic officer ( that is , a medical , dental or veterinary officer , a justice officer or a religious officer ) , is equivalent to a professional officer of the second class in the reserve and equivalent to first lieutenant . United Kingdom . British Army . In the British Army and Royal Marines , the rank above second lieutenant is simply lieutenant ( pronounced " Lef - tenant " ) , with no ordinal attached . Before 1871 , when the whole British Army switched to using the current rank of " lieutenant " , the Royal Artillery , Royal Engineers and Fusilier regiment s used " first lieutenant " and " second lieutenant " . Royal Navy . The first lieutenant ( often abbreviated " 1st Lt " ) in a Royal Navy ship is a post or appointment , rather than a rank . Historically the lieutenants in a ship were ranked in accordance with seniority , with the most senior being termed the first lieutenant and acting as the second - in - command , unless the ship was complemented with a commander . Although lieutenants are no longer ranked by seniority , the post of " first lieutenant " remains . In minor war vessels , destroyer s and frigate s the first lieutenant is second in command , executive officer ( XO ) and head of the executive branch ; in larger ships where a commander of the warfare specialisation is appointed as the executive officer , a first lieutenant is appointed as his deputy . The post of first lieutenant in a shore establishment carries a similar responsibility to the first lieutenant of a capital ship . Colloquial terms in the Royal Navy for the first lieutenant include " number one " , " the jimmy " ( or " jimmy the one " ) and " James the First " ( a back - formation referring to James I of England ) . United States . U . S . Army , U . S . Marine Corps and U . S . Air Force . In the United States Army , U . S . Air Force , and U . S . Marine Corps , a first lieutenant is a junior commissioned officer . It is just above the rank of second lieutenant and just below the rank of captain . It is equivalent to the rank of lieutenant ( junior grade ) in the other uniformed services . Promotion to first lieutenant is governed by Department of Defense policies derived from the Defense Officer Personnel Management Act of 1980 . DOPMA guidelines suggest all " fully qualified " officers should be promoted to first lieutenant . A second lieutenant ( grade O - 1 ) is usually promoted to first lieutenant ( grade O - 2 ) after 18 months in the Army or 24 months in the Air Force and Marine Corps . The difference between the two ranks is slight , primarily being experienced and having higher pay . It is not uncommon to see officers moved to positions requiring more experience after promotion to first lieutenant . For example , in the Army and Marine Corps these positions can include leading a specialty platoon , or assignment as the executive officer for a company - sized unit ( 70 - 250 soldiers or marines ) . In the Air Force , a first lieutenant may be a flight commander or section ' s officer in charge with varied supervisory responsibilities , including supervision of as many as 100 + personnel , although in a flying unit , a first lieutenant is a rated officer ( pilot , navigator , or air battle manager ) who has just finished training for his career field and has few supervisory responsibilities . U . S . Navy and U . S . Coast Guard . In the U . S . Navy and Coast Guard , first lieutenant is a position title , instead of a rank . It is held by the officer in command of the deck department . On smaller ships , the first lieutenant holds the rank of lieutenant , junior grade or ensign . On larger vessels , the position is held by a lieutenant or , in the case of extremely large warships such as aircraft carrier s , a lieutenant commander or even commander . However , on submarine s and in aircraft squardrons , where the deck department may only have a few junior sailors , the first lieutenant may be an enlisted member , such as a first - class petty officer or chief petty officer . The 1st Lieutenant Division is usually composed of junior sailors ( E - 3 and below ) who are completing their 90 days of Temporary Assigned Duty , or TAD , that all enlisted personal are required to do when initially assigned to a command . The primary mission of the division is servicing , cleaning , organizing and inventorying items within a command . Other countries . For other countries , the equivalent rank to a US Army first lieutenant ( O - 2 ) is listed below .