The phenol tolerances of three Alabama populations of mosquitofish Gambusia affinis were compared; two populations lived in industrially polluted streams and one in a nonpolluted lake. In 48-hour static tests, the median lethal concentration (LC50) of phenol was significantly higher for mosquitofish from a stream that received coke-treatment wastewater from a steel plant than for the other two populations. Especially at low phenol concentrations, most mortality occurred during the first few hours of exposure, indicating that some individuals were much more susceptible to phenol than others. Eight-hour assays of fish from the lake and the most polluted stream confirmed the presence of distinctly phenol-resistant and phenol-susceptible groups of fish. The differences in LC50 values for these two populations reflect differing proportions of resistant individuals. Resistant fish made up 67-80% of the polluted-stream population with the high LC50 and only 23-27% of the nonpolluted-lake population. Repeated nonlethal toxicity tests on individuals demonstrated that some fish are invariably susceptible and others show resistance at some times and susceptibility at others. Phenol resistance apparently depends on the induction of a genetically determined, rapidly acting detoxification mechanism. © 1983 by the American fisheries society.