The writer spent the period May 1 to October 19, 1948, in a temporary laboratory beside the Au Sable River in Crawford County, Michigan, where he maintained continuous observations on the aquatic insect fauna. Among a number of novelties obtained during the course of this study was the Baetis described below. Nymphs first attracted attention in collections made in mid-August. They grew rapidly thereafter, and subimagoes began to emerge from the stream and from laboratory rearing containers on September 11. Emergence was heaviest during the period October 1 to 3; no adults were taken after October 13.