Larvae of Culex quinquefasciatus Say and Cx. tarsalis Coquillett resistant to organophosphates by means of esterases were found to be fully susceptible to 26 synthetic pyrethroid insecticides that were examined [resistance ratios (RR) = 0.24 to 1.7]. However, Cx. quinquefasciatus resistant to propoxur and DDT, and Anopheles albimanus Wied. resistant to organophosphates, carbamates and DDT, were found to possess a low level of cross-tolerance to some of the pyrethroids. The available evidence indicates that this tolerance is due largely to the DDT resistance component of the strains.