A synthetic pyrethroid, SD43775 [Benzeneacetic acid, 4-chloro-alpha-(I-methylethyl), cyano (3-phenoxyphenyl) methyl ester], showed relatively high activity on 2-day old mosquito fish, Gambusia affinis (Baird and Girard), when applied at rates used for mosquito control. Populations of plankton crustaceans and mayfly nymphs were also affected but recovered within a short period of time. Populations of Corixid, Notonectid, and aquatic beetles were slightly affected, and the effects were sustained throughout the experiment, Chironomid larval populations were suppressed and emergence inhibited. No deleterious effects were observed against, rotifer populations.