The toxicity of 44 freshwater effluents was evaluated using the fathead minnow (Pimephales promelas), a cladoceran (Ceriodaphnia dubia) and Microtox (Photobacterium phosphoreum). The latter assay was conducted with both sodium chloride (NaCl) and sucrose for osmotic protection of the bacteria in an effort to determine whether the use of sucrose increased the sensitivity of Microtox to samples exhibiting toxicity to the fish and cladoceran species. Twenty-three of the effluents tested were toxic to fathead minnows and/or C. dubia; of these, Microtox was sensitive to 10 when tested in NaCl and seven when tested in sucrose. In one instance testing in sucrose predicted toxicity to the fish and cladoceran species when testing in NaCl did not. Single chemical tests demonstrated that Microtox sensitivity was greater to zinc chloride, nickel chloride and methanol when tested in sucrose as opposed to NaCl, about equal to copper sulfate and phenol in sucrose and NaCl, and was far greater to chlorine when tested in NaCl than in sucrose. These results, as well as data from Microtox assays with effluents containing known toxicants, suggest that the use of sucrose, in conjunction with NaCl, for osmotic adjustment in the assay could be helpful as part of a suite of tests for identifying general classes of compounds responsible for toxicity in freshwater effluents.