Toxicity screening tests of 10 days duration, using microcosms in a experimental pond, were performed. {A figure is presented}-BHC was applied at different concentrations as a model chemical to evaluate the toxic effects to natural occuring plankton. Zooplankton taxa showed sensitive reactions to the chemical perturbation even at 20 μg·L-1. The methodology developed proved to be a helpful means for roughly estimating the toxicity of micropollutants to planktonic freshwater organisms.