An unusual mortality event in the fall of 1999 was observed in Long Island Sound lobsters resulting in the reduction of up to 99% of lobster landings. 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. To determine the possible implications of pesticide 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-d or 28-d, and the direct toxicity, as well as sublethal effects on the immune system was determined. The 96-h LC50 for sumithrin upon single exposure, as well as the 28-d LC50 for sumithrin upon repeated exposure, were greater than 1.0 μg/L, the highest concentration tested. No modulation of immune function was detected after sumithrin exposure for either the single or repeated exposures. Water concentrations of sumithrin decreased rapidly over the 5-d single sumithrin exposure, with a half-life of less than 2-h, 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 killed 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.