During the course of investigation of the mayfly fauna in streams in Ontario and Quebec it became apparent that many of the nymphs of the Baetinae are not known although the adults of most have been named. A few nymphs are described in scattered papers in the literature and a key has been made for some Baetis and Pseudocloeon (McDunnough 1932, 1932a) which satisfactorily separates groups within these genera. It would seem that in work on streams in the future these small mayflies would become increasingly important because of their abundance and wide distribution.