This study determined the toxicity of parathion and methyl parathion to organochlorine compound-resistant and -susceptible populations of mosquitofish (Gambusia affinis). Mosquitofish can tolerate about 40 times more methyl parathion than parathion. The resistant population demonstrates a 1.3 fold greater tolerance of methyl parathion than the susceptible population, which may be a developing resistance. The resistant population also demonstrates an environmentally-induced tolerance to parathion (1.6 fold) in the spring but no overall tolerance to parathion in the fall when compared to the susceptible population.