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Adult grass shrimp ( Palaemonetes pugio ) were exposed to endosulfan or methoprene for 96 h and LC 50 values were calculated. Male and female P. pugio cohorts were also exposed to endosulfan for 96 h in an attempt to determine potential differences in sensitivity between the sexes. Results from the methoprene exposure indicated that this pesticide was not acutely toxic to adult grass shrimp at 1 mg l −1 . Due to the lack of sensitivity, sex specific tests with methoprene were not performed. The calculated LC 50 for a population of grass shrimp, including both males and females exposed to endosulfan, was 0.62 μg l −1 . The LC 50 determinations for the sex specific tests were 0.92 μg l −1 for males and 1.99 μg l −1 for females. Following these acute exposures, reproductively active grass shrimp were chronically exposed to 200 ng l −1 endosulfan or 1 mg l −1 methoprene and were allowed to produce embryos. The resulting embryos were assessed for potential sublethal toxicity. There were no observed differences in the percent successfully hatching or larval mortality 3-days post hatch among the treatments. However, endosulfan treated embryos had a significantly increased hatching time (9.76 days compared to 8.72 days in controls). Methoprene treated embryos also took longer to hatch (9.55 days), but this delay was not significantly different from controls. These findings suggest that endosulfan may preferentially affect male grass shrimp and exposed female grass shrimp may produce embryos with delayed hatching times.