The 96-h LC 50 values (with the 95% confidence limits in parentheses) of fenitrothion for the freshwater fish, Mystus cavasius and three size groups of Labeo rohita (3 to 4·4 cm, 4·5 to 5·9 cm and 6 to 8 cm in length) were respectively 3·3 (3 to 3·5), 2·8 (2·6 to 3), 4·1 (4 to 4·2) and 4·6 (4·5 to 4·7) ppm. There was a decrease in the toxicity of the compound with increase in size, and a significant negative correlation between increasing concentrations of fenitrothion and the oxygen uptake of L. rohita. Although it is less toxic to mammals than the closely related methyl parathion, fenitrothion seems to be more toxic to fish.