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 female looks very much like a male I collected a few hundred miles away a few days later, so I'm guessing it's the same species, which I believe is Maccaffertium mediopunctatum.
This mayfly was collected from the Little Juniata River in Pennsylvania on May 23rd, 2007 and added to Troutnut.com by Troutnut on June 4th, 2007.