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 is a really really really really tiny mayfly nymph. It has gills on abdominal segments 3-7 in the characteristic orientation for the Ephemerellidae family, and it has no fain tail as best I can tell, just uniform tail coverage with small black hairs of equal length. I think this one's too young to identify.
This mayfly was collected from unknown in Wisconsin on March 10th, 2004 and added to Troutnut.com by Troutnut on January 19th, 2006.