The report reviews the laboratory toxicity data on fresh water target and nontarget invertebrates exposed to pyrethroids and evaluates the state of the data. Early in their development, pyrethroids were shown to be highly effective insecticides against mosquitoes, blackflies and chironomids. Acute LC50 values for these target animals were often less than 1 micro g/L. Acute exposures of nontarget invertebrates showed that some species were as sensitive as the target insects. Longer exposures, up to 28 days, showed that lethal effect concentrations were lowered with increased exposure duration. Unfortunately, the long-exposure tests were limited to a few species and a few pyrethroids, so no general commentary on no effect concentrations is possible. Some species were affected at concentrations near the analytical detection limit for pyrethroids. The data reviewed show that temperature, life cycle stage and exposure duration will be critical factors in developing the no effect concentrations that are important in risk assessments for nontarget invertebrates exposed to these insecticides.