Toxicity tests using early life stages of fish are of great importance in assessing risks to growth, reproduction and survival in polluted environments and are important tools for good environmental monitoring. However, a small number of standard bioassays of this type have been developed in Brazil. Curimbatá (Prochilodus lineatus) is an abundant South American characid fish of great commercial interest. It was chosen for testing different concentrations of 40% dimethoate, an organophosphate insecticide and acetylcholinesterase enzyme (AChE) inhibitor used widely in Brazil. The 48-h LC50 for eggs is higher than 16.0 µg.L-1, whereas for recently hatched larvae it was found to be significantly lower (11.81 µg.L-1, ranging between 10.23 µg.L-1 - 13.65 µg.L-1) and also significantl y lower than that for eggs by a Student t-test for independent samples (p = 0.03). The 96-h LC50 for 3-day old larvae was 10.44 µg.L-1 (8.03 µg.L-1 - 13.57 µg.L-1), similar to that of recently-hatched larvae (p = 0.76). Larval mobility was also found to be reduced by this insecticide.