The authors conducted a series of laboratory pulse-exposure experiments to model short-term field exposures of two representative estuarine crustaceans, Penaeus duorarum and Mysidopsis bahia, to the organophosphate insecticide fenthion. These tests established acutely lethal and nonlethal concentrations during pulse exposures. The authors varied in situ exposure regimes and obtained responses among caged test populations that ranged from no observed effect to 100% mortality. The responses of caged pink shrimp and mysids exposed to slowly changing concentrations of fenthion in the field were similar to what would have been predicted based on laboratory tests that established 24-, 48-, and 72-h LC50's.