The study was conducted to determine the effect of pupal chemosterilization on the susceptibility of the resulting Anopheles albimanus and A. freeborni mosquitoes to Plasmodium vivax. Mosquitoes were sterilized by exposing pupae for 1, 2 or 4 hours in 1% aqueous solution of the thioaziridine agent, P,P-bis(1-aziridinyl)-N-methyl phosphinothioic amide, ENT-61585. Anopheles albimanus and A. freeborni responded differently to pupal treatment and subsequent adult infection. Virtual complete steriligy of A. albimanus and A. freeborni was obtained after 1- and 4-hour exposure of the respective pupae to the sterilant. The difference in susceptibility between sterilized and normal mosquitoes to the malaria infection was more pronounced in A. albimanus than in A. freeborni. The overall infection rate of treated A. albimanus was significantly reduced over that of the control mosquitoes. In A. freeborni, however, a less marked effect was noticed. The infection in sterilized mosquitoes was not eliminated and sporogony of P. vivax was completed with motile sporozoites in the salivary glands.