Acute and chronic toxicity tests with diazinon (diethyl 2‐isopropyl‐6‐methyl‐4‐pyrimidinyl phosphorothionate) were conducted onDaphnia magna.The 24‐hr static LC50 was 0.86 μL.L‐1. The sublethal effects of 0.05, 0.1, 0.5, 0.75 and 1.0 ngL‐1of diazinon concentrations on the survival, reproduction and growth ofD. magnawere monitored for 21 days. The algaeNannochloris oculata(5 x 105cellsmL‐1) was used to feed the daphnids. The parameters used to determined the effect of the pesticide onD. magnawere: mean total young per female; mean brood size; days to first brood; intrinsic rate of natural increase (r); growth; and survival. Reproduction as well as survival was significantly reduced at diazinon concentrations of 0.10 ngL‐1and higher. The intrinsic rate of natural increase (r) decreased with increasing concentrations of diazinon. Growth, as measured by body length, was depressed significantly at 0.05 ngL‐1of diazinon and higher concentrations. The maximum acceptable toxicant concentration (MATC) was calculated. The chronic data was used to formulate an acute/chronic ratio.