The static acute toxicities of alpha-endosulfan, beta-endosulfan, (alpha + beta)-endosulfan, endosulfan sulfate (their transformation product), and formulated materials were determined for a representative freshwater amphipod (Hyalella azteca), cladoceran (Daphnia magna), and salmonid fish (Oncorhynchus mykiss). Acute lethality tests also were conducted on these organisms and coho salmon (Oncorhynchus kisutch) by exposing them to simulated field water endosulfan concentrations, using either a single compound and/or in combinations. As well, growth/survival bioassays of H. azteca were carried out in simulated sediment endosulfan concentrations that were detected in farm ditches contiguous to fish streams in the Lower Fraser Valley (BC, Canada). All materials tested were very highly toxic to these nontarget aquatic indicator organisms. As the most potent compound, alpha-endosulfan is about 1.3, 6.6, and 58 times more toxic than the beta-isomer to Daphnia, rainbow trout, and Hyalella, respectively. Some possible biological and toxicological implications of our findings are discussed in the context of endosulfan concentrations found in farm ditches flowing to fish streams in the Lower Fraser Valley.