the effects of chlorpyrifos (Organophosphate insecticide) on Zooplankton were studied in outdoor experimental tanks (mesocosms) sited in the Mediterranean Region (Madrid, spain) at two nominal concentrations of chlorpyrifos (0.1 and 1 microg a.s./L applied as chas 48) and control were used five tanks were used as control and the treatments were performed in quintuplicate. A single chlorpyrifos application simunlating spray-drift was conducted. the population and community effects were analyzed by means of univariate statistics and through the multivariate principal response curves technique