Larvae of the estuarine grass shrimp, Palaemonetes pugio, were reared in the laboratory from hatch through metamorphosis under optimal salinity conditions in a range of lethal and sublethal concentrations of the pyrethroid insecticide, fenvalerate. A nominal concentration of 3.2 ng fenvalerate/l significantly reduced the percentage of larvae that completed metamorphosis. Oxygen consumption rates were significantly higher for larvae exposed to this lethal concentration for 24 hr. Exposure to a sublethal concentration of 1.6 ng fenvalerate/l prolonged the duration of complete larval development. After 8-day exposure to fenvalerate, oxygen consumption rates were elevated when larvae were exposed acutely to hypo-osmotic stress. Alterations in metablic-salinity patterns of larval grass shrimp developing under sublethal concentrations of fenvalerate suggest reduction in the ecological fitness during this life stage by limiting capacity of larval shrimp to adapt to the fluctuating salinity conditions of estuarine waters.