Abstract 1. The effect of short-term exposure to the fungicide copper sulfate, on the feeding behavior of two freshwater zooplankters, the rotifer Brachionus calyciflorus and the cladocera Daphnia magna, was studied. 2. Both species were exposed to sublethal levels (0.01, 0.02, 0.05 and 0.08 mg/l) of copper sulfate during 24 hr in order to determine the effect of this toxicant on filtration and ingestion rates. 3. The experiments were performed using the unicellular algae Nannochloris oculata in a density of 5 × 10 5 cells/ml, as food for both organisms. 4. Rates of filtration and ingestion declined on B. calyciflorus and D. magna with increasing toxicant concentrations. 5. The effective copper sulfate concentration at which feeding rate was reduced to 50% of that in controls (ec 50) was 0.043 and 0.090 mg/1 for B. calyciflorus and D. magna, respectively.