Forty-nine species of Entomostraca were found in 48 lakes situated in the southern half of Saskatchewan. These lakes form a series ranging from fresh to extremely saline waters. The salinity in most cases was less than 10,000 p.p.m. The occurrence of Entomostraca is correlated with the degree of salinity. Total concentration is considered to be of prime importance in determining presence or absence of species. There were pronounced differences in the total salt content of waters inhabited by the various species. Cladocera were more markedly restricted by high salinity than were Copepoda. Littoral species were apparently more susceptible to increasing concentration than were those of limnetic habits. Typical entomostracan communities in these lakes are discussed. The communities in highly saline waters are quite comparable to those reported in similar habitats in other parts of the world. Marked alterations in the fauna of two lakes have resulted from salinity changes over a period of 20 years. Tentative upper limits of salinity tolerance for many of the species have been proposed. Salt concentrations of between 4000 and 16,000 p.p.m. appear to be upper limits for most freshwater types. The majority of species are considered to possess no special osmoregulatory mechanisms. Except for Artemia salina and Moina hutchinsoni the Entomostraca present in these lakes are typically freshwater species which in general have a wide geographic distribution. The fauna has undoubtedly been derived from many sources and in only a few instances are there indications of definite points of origin.