During the summers of 1928 and 1929 a study was made of the mayfly fauna of several streams and lakes in Ontario, particular attention being given to associating the nymphal stages with the adults by rearing them in cages set in the stseams. In the course of the work the nymphs of twelve species, hitherto unknown in this stage, were found. These are described below. Three new species were taken and are also described.