Aedes sollicitans (Walker) (Diptera: Culicidae) is an important vector of eastern equine encephalitis as well as several other mosquito-borne brain fevers. The larvae are salt-tolerant and develop in salt marshes with highly varying salinity. The effect of salinity on the toxicity of one of the major larvicidal organophosphates, temephos, was evaluated in two groups of larvae raised either in freshwater or water with salinity ranging from 1 to 3.5%. When larvae were raised in freshwater, low salinity (1–3.5%) decreased the toxicity and high salinity (5%) increased the toxicity. In contrast, salinity did not change the toxicity to larvae raised in saltwater. Temephos treatment and salinity seemed to have cross-interaction for the larvae raised in freshwater. High salinity also caused reduction in larval body size, and 5% salinity alone caused mortality for larvae raised in freshwater, suggesting that preadaptation to saltwater in the early instars is essential for survival in later instars at high salinity.