Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.
This male was collected at the same time as this female and is likely the same species.
It keys pretty clearly to Rhithrogena undulata using the key in Traver 1935, although the size is larger than expected for that species in that source.
This mayfly was collected from the Madison River in Montana on July 1st, 2019 and added to Troutnut.com by Troutnut on July 18th, 2019.