Acute toxicity tests with selenium were conducted with three freshwater species. All data are expressed as selenite-selenium. Daphnia pulex had a 48 hr LC50 of 3.87 mg/l selenium. The 96 hr LC50 and EC50O values for Gambusia affinis and Physa sp. respectively, were 12.56 and 27.08 mg/l selenium. The sublethal effects of 0.2, 0.4, 0.6 and 0.8 mg/1 selenium on survival, growth and reproduction of Daphnia pulex were monitored for twenty-eight days. These results were analyzed statistically by brood. Appreciable mortality only occurred at 0.8 mg/l selenium. Growth, as measured by body length, was depressed at the highest concentration during the early instars and was slightly stimulated during the later instars. Number of live young per brood was depressed at 0.4, 0.6 and 0.8 mg/l selenium during the early broods and may have been stimulated in later broods. Reproductive dysfunction (i.e., dead young, deteriorated eggs, and abortions) only was significant at the higher concentrations in the early broods. It appeared that the Daphnia were acclimating to the selenium stress. Based on these studies, the MATC for selenite-selenium was 280 ug/l. The effects of selenium on oxygen consumption and filtering rate during 24 hr exposure were also tested at the above concentrations. There were no significant effects of selenium on oxygen consumption. Selenium slightly stimulated filtering rate at 0.2 mg/l and depressed it at the higher concentrations. An evaluation of the water quality criteria for selenium in fresh water indicates that all of the methods for deriving these proposed criteria are inadequate. Based on my evaluation a dual criterion seems appropriate: a strict criterion for selenate-selenium and lenient criterion for selenite- and biselenite-selenium. / Master of Science