Abstract: Chronic toxicity of fenvalerate, beta-cypermethrin, chlorpyrifos and triazophos towards water flea, Daphnia magna, was tested in a 21 d test. Chlorpyrifos of ≥0.5 μg/kg and fenvalerate and beta-cypermethrin of ≥0.05 μg /kg significantly prolonged the days to first pregnancy and the days to first breeding, while triazophos had no impact on the above two parameters in range of concentrations tested (0.05~2.0 μg /kg). Compared with the days to first pregnancy and the days to first breeding, parameters such as the number of first brood, the total number of offspring and the total number of broods laid during the 21 d period, intrinsic rate of natural increase(r) seemed more sensitive to insecticide exposure. At the end of 21 d exposure the body length was not affected by chlorpyrifos and triazophos, while fenvalerate and beta-cypermethrin of ≥0.025 μg/kg significantly degraded the length of the body of the daphnia. Taking all the tested parameters into consideration itcoued be gound that the non-effective concentration (NOEC) of fenvalerate, beta-cypermethrin, chlorpyrifos, and triazophos were 0.005, 0.005, 0.01, 0.05 μg/kg, respectively. In contrast with the organophosphorus insecticides, the body length could serve as a special index of pyrethroid exposure. Integrating 48 h LC50s with the corresponding NOEC, the application factor (AF) of the 4 insecticides in range of 0.004~0.012 could be calculate. The present study suggests that insecticide contamination results from spray draft and run-off may cause both acute and chronic hazards on sensitive organisms, such as daphnia, in aquatic ecosystem. Since large diversity exists among the AFs it is better to undergo actual longer-term experiment, rather than by prediction, to determine the impact of certain insecticide on population dynamics of daphnia.