A comparative aquatic hazard assessment of resmethrin was conducted to investigate the need for its restricted use classification by the US. EPA as an adult mosquito control agent. This paper describes the environmental fate and aquatic toxicity of resmethrin. The following paper compares resmethrin to the alternative insecticides. Environmental fate studies indicate that resmethrin has a short photolytic half-life in water (<1 h). Furthermore, it is immobile in soil and biodegradable (half-life = 36.5 d) under aerobic conditions. Laboratory studies with constant 48- to 96-h exposures show it is acutely toxic to fish and invertebrates in the 0.22-15.0 microg/L range. Daphnia magna, pink shrimp (Penaeus duorarum) and rainbow trout (Oncorhynchus mykiss) are the most sensitive and mollusks are the least sensitive species. Chronic laboratory studies indicate that the maximum acceptable toxicant concentrations (MATCs) for resmethrin and D. magna, Pimephales promelas, O. mykiss, and Cyprinodon variegatus are 0.58, 0.52, 0.43, and 10.3 microg/L, respectively. The acute-to-chronic ratios (1.1-7.3) for all species studied indicate that chronic toxicity will not be an issue for resmethrin. Furthermore, the characteristics of acute exposures (48- to 96-h) used in the laboratory will not occur under field conditions because of the short half-life of resmethrin in fresh- and salt-water.