Interspecies comparisons indicate that fish are relatively more resistant to acute intoxication with parathion and paraoxon than are rodents. In contrast, fish are more sensitive to malathion and malaoxon. The following investigation was designed to determine if species-related differences in the sensitivity of brain acetylcholinesterase (AChE) to inhibition by paraoxon and malaoxon could contribute to the interspecies differences in toxicity. Brain AChE activity was significantly greater in fathead minnows and rainbow trout than in rats and mice. The fathead minnow and rainbow trout IC50 values for paraoxon were 228- to 1879-fold greater than the corresponding values for rat and mouse. Similarly, the Ki (bimolecular inhibition constant) was 159- to 1663-fold greater in rodents than in fish, which reflected both a higher KA (association constant) and kp (phosphorylation constant) in rodents. The rodent IC50 values for malaoxon were 30-80% that of the fish IC50, and the Ki was 30-50% greater in rodents than in fish. These data suggest that the greater sensitivity of rodent brain AChE to inhibition by paraoxon may contribute to the greater toxicity of parathion and paraoxon in rodents than in fish. In contrast, the lack of correlation between the inhibition of brain AChE by malaoxon and species-related differences in acute LD50 suggests that other factors, such as the limited carboxylesterase activity in fish, may be responsible for this species selectivity.