A comparison of the susceptibility status of adults of Anopheles albimanus from Panama and El Salvador showed the former to be highly susceptible to all the insecticides tested. In contrast the El Salvador population showed a broad spectrum of resistance involving DDT, propoxur, and a number of organophosphates but remained susceptible to fenthion and the 2 pyrethroid insecticides, permethrin and decamethrin. Exposures of the resistant population to combinations of malathion and the synergists TPP, PB, SV, and DEF produced results suggestive of the involvement of at least 2 detoxication mechanisms, carboxylesterase and mixed function oxidases.