Most toxicity information available for evaluating potential effects of mosquito control chemicals on nontarget aquatic biota comes from acute lethality tests of 24- to 96-hr duration. These studies generally show that insecticides are more toxic to aquatic invertebrates than fishes. Crustaceans, in particular, are extremely sensitive to mosquito control insecticides, perhaps a result of their close phylogenetic relationships with insects. Effects of longer-term exposures on survival and growth or studies that quantify other sublethal effects are available only for selected, standard laboratory test species for some chemicals. Field studies conducted by the author's laboratory following operational insecticide applications have shown that exposure can be shorter duration and of lesser concentration than those used for worst-case scenarios in screening level environmental risk assessments. However, long-term effects of repeated applications of the same chemical or cumulative effects of multiple-chemical treatments have not been adequately assessed in the field.