They are found in both the western and eastern United States, and in Europe, too. This is one group of mayflies whose Latin name has become widely used by the angling public. They are also called blue-winged olives, or just olives. In the eastern United States and England, other species are called blue-winged olive, so Baetis is a more reliable term. Many other mayfly species are known by their common names rather than the Latin.
Baetis mayflies are the season's first and last mayfly hatch.
They often appear dark gray on the water. Baetis hatch from about noon to late afternoon or early evening in spring. This is the warmest time of day then. Despite that, they prefer overcast, humid conditions to hatch in. Cloudy, rainy, even snowy days bring out the best hatches.