Two strains of Anopheles albimanus Wied, and five strains of Culex quinquefasciatus Say representing the principal mechanisms of resistance to organophosphorus, carbamate, DDT, dieldrin and pyrethroid insecticides were tested for cross-resistance to Bacillus thuringiensis var. israelensis toxin. Maximum interstrain differences in LC90 upon 48-hr exposure of 4th instar larvae were 1.9x for Culex and 1.5x for Anopheles, indicating that presently-existing resistance to insecticides should not significantly affect the larvicidal activity of this bacterial toxin.