Twelve collections of Culex quinquefasciatus from separate areas of Galveston County, Texas and 8 from various parts of Brazoria Country were established as temporary strains in the laboratory for 2 or 3 generations to provide sufficient material for insecticide susceptibility tests. As compared with a standard susceptible strain (UTMB), the LC50's for malathion of 9 strains of larvae from Galveston County ranged from 0.0058 to 0.280 ppm or a maximum of 7X less susceptible. The LD50's of the adults ranged from 0.9% to > 5.0% malathion. Five strains of larvae from Brazoria County were 2X to 9X more tolerant to malathion than the standard, whereas the LD50's of the 8 strains of adults ranged from 1.3% to >5.0%. Larvae from both counties exhibited tolerance to chlorpyrifos, ranging from 2X to 13X as compared with a susceptible strain. The larvae of 2 strains tested against a battery of other organophosphorus insecticides not used in mosquito control, exhibited decreased susceptibility which in the case of fenitrothion and parathion reached 16X and 20X, respectively, less susceptible than the UTMB strain.