An estuarine mysid (Mysidopsis bahia) has been identified as one of the most sensitive members of the estuarine community to pollutant stress. In the majority of life-cycle toxicity tests using this planktonic estuarine crustacean, sublethal reduction in reproductive potential is the most sensitive criterion for chronic biological effect. Recent information suggests that retarded juvenile growth rates and alterations in energy metabolism of exposed mysids precedes reductions in reproductive capacity. Results of these laboratory studies, confirmed recently in a field study, indicate that measurements of metabolic dysfunction in mysids exposed to microcontaminants may be used to predict altered production rates in these sensitive crustacean populations.