We studied the effects of five different concentrations of copper (0.05, 0.20, 0.45, and of 0.05 added continuously up to 0.45 mg l-1) in 30 l aquaria containing water and organisms from Presidente Calles reservoir, the biggest reservoir in the State of Aguascalientes, Mexico. Large reductions in zooplankton density, species diversity and total chlorophyll were found when we compared the different treatments against controls. Initial addition of a greater amount of copper has a more adverse effect than constant additions of small amounts of copper as suggested by smaller reductions in zooplankton density and chlorophyll. This result agrees with the previous reports on the effects of other metals in different aquatic microcosms. The most dramatic effect of copper addition on zooplankton density was found three days after copper addition. Lethal effects on cladocerans, copepods, and rotifers were found at a concentration of 0.45 mg l-1. These results are important for regulatory agencies in Mexico as there are only a few studies about effects of toxicants on Mexican aquatic ecosystems.