Laboratory bioassays were conducted to assess the tolerant limits of forth instars larvae of field and laboratory strains Aedes aegypti and Culex quinquefasciatus. These mosquito larvae were exposed to varying pH regimes and quantal mortality were assessed after 24 hours. The result indicated that maximum survival of both field and laboratory strains occurred between the pH values of 6.5 and 8.0. Outside these range the mosquito larvae suffered high mortalities in 24 hours of exposure. The responses of field and laboratory strains of both Aedes aegypti and Culex quinquefasciatus were similar, indicating that pH was not exerting any selection pressure on the field strain in the local environment.