While collecting at Birch Glen in Logan Canon, Utah, on June 18th, 1926, I first came upon the nymph of the mayfly (Rhithrogena mimus Eat.) that is the subject of this article. I was gathering aquatic insects from among the larger stones in a swift portion of the river. Dipterous larvae of Simulium and Bibiocephala were common, and the nymphs of the big mayfly Ephemerella doddsi (locally called the “ginger quill”) were appearing on my hand screen, when along with them appeared a few peculiar mayfly nymphs with broadly expanded gills of a bright rose-red color. I quickly got some of them into a rearing cage and in a few days had sub-imagos, and later adult reared specimens, of the species described below.