An organophosphorus-resistant strain of Culex quinquefasciatus was pressured with pyriproxyfen for 17 generations. Egg viability began declining in the F7 generation and became lower as the selection process continued; by the F17 generation egg viability was too low to proceed further. Susceptibility tests on larvae of the F5, F10, F15, and F17 generations showed no indication of increased tolerance to pyriproxyfen.