Survival, growth, and various measures of reproductive performance were examined for an estuarine mysid, Mysidopsis bahia, throughout its life cycle during exposure to the organophosphate insecticide, fenthion. Both individual fecundity of females and total population production of young were reduced by fenthion concentrations of 79 ng/l and higher. Suppression of mysid growth rates was evident after only 4 days exposure of juvenile mysids to sublethal fenthion concentrations; lower concentrations (79 ng/l) retarded growth rates of the more rapidly growing advanced juveniles after approximately 2 weeks exposure. Reduced survival capacity, retarded growth rates, and diminished reproductive success of mysid populations with chronic, low-level exposure to fenthion would result in lowered production rates of an important prey population for commercially important fish that utilize the estuary as a nursery.