A lobster die-off significantly reduced the 1999 fall landings in western Long Island Sound by up to more than 99%. The die-off corresponded in time with the application of pesticides for the control of mosquitoes that carried West Nile virus, an emerging disease in North America at that time. Two pesticides used to control mosquitoes that carried West Nile virus, resmethrin and malathion, have recently been documented to reduce lobster hemocyte phagocytosis, an important innate immune response necessary to eliminate invading pathogens, upon experimental exposure. However, the immunotoxicity of sumithrin has not been documented in lobsters. In order to determine the possible implications of sumithrin application as a direct or contributing factor in the die-off, studies were conducted to determine the effects of experimental exposure to sumithrin on the health of lobsters. Lobsters were exposed in 20-gallon tanks for 5 or 28 days, and the direct toxicity, as well as sub-lethal effects on the immune system were determined. The 96-hour LC50 for sumithrin upon single exposure, as well as the 28-day LC50 for sumithrin upon repeated exposure, were greater than 1.0 µg/L, the highest concentration tested. No modulation of immune function was detected following sumithrin exposure for either the single or repeated exposures. Water concentrations of sumithrin decreased rapidly over the 5-day single sumithrin exposure, with a half-life of less than 2 hours, with similar decay rates for all concentrations tested. Overall, this study showed that sumithrin degrades rapidly in salt water and did not exert significant toxicity in lobsters (it neither killer lobsters nor induced immunotoxicity) at the concentrations tested. It is unlikely that sumithrin significantly contributed to the 1999 lobster die-off in Long Island Sound.