Grazing experiments were carried out in a limed lake (Lago d'Orta) during the periods, 23–24 March and 28–29 June 1990. A simple in situ technique based on cell counts before and after incubation was used in order to evaluate the impact of the natural zooplankton community on the lake phytoplankton. An estimation of the daily consumption by the natural zooplankton community showed that about 4.6% and 1.04% of the standing algal biomass was removed during March and June respectively. The differences in grazing activities were likely due to the difference in zooplankton community structure and to the surrounding environmental conditions, mainly influenced by the liming intervention, started in the lake water since 1989.