ABSTRACT: Organophosphate insecticides can compromise water quality and harm non-target species unintentionally. In this study, we examined the effects of a commonly used organophosphate insecticide, Malathion, on blue crabs Callinectes sapidus, which are a an economically and ecologically important estuarine species. Adult and juvenile crabs were exposed to environmentally-occurring concentrations of Malathion at 2 salinity levels to determine how these factors would affect crab behavior and mortality. Initial Malathion concentrations of 1.0 ppb and 11.2 ppb caused a significant increase in mortality of juvenile blue crabs, and concentrations of 11.2 ppb caused a significant increase in mortality of adults, all within 36 h after exposure. Our results indicated that salinity did not affect blue crabs’ susceptibility to Malathion at these concentrations. We measured the time needed for adult blue crabs to right themselves when placed on their backs before and after exposure to Malathion. After 1 h of exposure to Malathion concentrations of 11.2 ppb, adult crabs took significantly longer to right themselves, indicating that short-term exposure to Malathion at ecologically relevant concentrations can interfere with essential behaviors of blue crabs. Blue crab populations have been declining in recent years, and their decline has primarily been linked to overfishing. However, other mechanisms including pollution and disease may also contribute to this problem and require further investigation. Since blue crabs are important components of estuarine food webs, changes in blue crab mortality rates and behavior may have important consequences for entire estuarine systems.