In many parts of the northern tier of the U.S., the ephoron leukons mayfly is coming out in force, generally in August. The hatches can start as early as mid July and extend into September in many areas, depending on a number of weather factors. 

When conditions are right, the White Miller (approximately a size #12) can be quite prodigious. Here is a very effective pattern I like, the White Wulff. 

This pattern works very well for several reasons. First and foremost, it floats quite well. Second, the calfhair tail looks like the shuck of the dun phase which often trail behind the spinners as they flutter about and wind up on the water. 

This pattern has been very effective on smallmouth bass that will set up in feeding lanes, just like trout. They are selective and will not move very far, so your casting must be accurate. 

If you haven't experienced this hatch, it's going on right now (end of August in Minnesota). Check with your local fly shop or peruse online forums to find places were you can get in on this late evening to dusk hatch. 

Bring a headlamp!