The Cladocera of Ontario were studied from material collected over a wide geographical area. In all 244 lakes and 33 ponds were sampled, some of them more than once.A method for making stained permanent preparations of Cladocera used in our study is described.The genus Daphnia is richly represented. Based on a study of this material we have raised Daphnia pulicaria Forbes from synonymy in the North American fauna. The common limnetic species have been demarcated more satisfactorily. A hitherto undescribed Daphnia species was recorded. Some systematic notes are also included on species of the genera Diaphanosoma, Ceriodaphnia, Eubosmina, and Bosmina.Seventy species of Cladocera were recorded in the present study. This brings the species recorded from Ontario to 78 species; 26 species are recorded from Ontario for the first time. While the limnetic species presently known probably represent a complete list many littoral forms have certainly not been recorded so far.The distribution of the various species in the different parts of Ontario is given. The relative frequency and dominance of species is discussed.