Mosquitofish, Gambusia affinis, from two populations differing greatly in insecticide tolerance were tested for their ability to avoid concentrations of DDT, endrin, toxaphene and parathion in a Y-maze. Both susceptible and resistant fish avoided endrin, toxaphene, and parathion. DDT was avoided by susceptible fish but not by resistant fish. Resistant fish avoided damaging concentrations (except DDT) better than did susceptible fish.