In a 5-year study, Culex pipiens quinquefasciatus Say larvae from 2 Louisiana mosquito control districts (MCD) developed varying degrees of tolerance to chlorpyrifos, malathion naled. Larvae collected in the Jefferson parish mosquito control district exhibited an approximate 2x decrease in susceptibility to the 3 compounds in 1975 when compared to base lines established in 1970. Larvae collected from septic ditches in the St. Tammany Parish mosquito control district were approximately 25x less susceptible to naled in 1975 when compared to data obtained in 1970 (Lc50 data). Susceptibility to malathion and chlorpyrifos decreased 28x and 5x, respectively, during the 5-year period. There was no evidence of increased tolerance to these chemicals in larval populations collected in the Orleans Parish MCD.