Evaluations of 2 formulations of a new insect growth regulator (IGR), MV678 (Stauffer Chemical Co.) against salt marsh mosquito larvae, Aedes sollicitans and Culex salinarius were conducted in small-scale field plots. Tests were also conducted to determine the toxicity of these formulations to salt marsh killifish (Fundulus heteroclitus) and grass shrimp (Palaemonetes pugio). The encapsulated formulation (1.5 S was determined in-effective in preventing emergence of mosquitoes in the range of 0.0005 to 0.75 Ib AI/A. In contrast, the emulsifiable concentrate formulation (4 E), at rates as low as 0.001 Ib AI/A was found to be 100% effective in preventing emergence of adult Ae. sollicitans and Cx. salinarius. This is ca. 96% less material than previously determined to be effective. Results also showed that lower dosages (0.001 Ib AI/A) of MV678-4 E caused delays in larval and pupal development. Experiments showed that MV678-4 E remains active in salt marsh water for 3 to 6 days following initial treatment. Only slight (<20%) cumulative mortalities were observed among killifish and grass shrimp exposed to MV678-4 E. Results suggest that MV678-4 E has potential as an alternative chemical control measure of salt marsh mosquito larvae.