Phosphorus (PO,-P) and nitrogen (NH,-N) excretion rates of Euchlanis dilatata lucksiana, a rotifer, isolated from Lake Loosdrecht (The Netherlands) and cultured in the lake water at 18-19 " C , were measured in the laboratory. In a series of experiments, the effects of experiment duration on the P and N excretion rates were examined. The rates measured in the first half-hour were about 2 times higher for P and 2-4 times for N than the rates in the subsequent three successive hours which were quite comparable. Eight experiments were carried out in triplicate, 4 each for P and N excretion measurements, using animals of two size ranges: 60-125 pm and > 125 pm. The specific excretion rates varied from 0.06 to 0.18 pg P.mg-DW.h-' and 0.21 to 0.76 pg N.mg-' DW.h-'. Generally an inverse relationship was observed between the specific excretion rates and the mean individual weight. The excretion rates of Euchlanis measured by us are lower than those reported for several other rotifer species, most of which are much smaller than Euchlanis. Extrapolating the excretion rates of Euchlanis to the other rotifer species in Lake Loosdrecht, and accounting for their density, size and temperature, rotifer excretion appears to be a significant, potential nutrient (N,P) source for phytoplankton growth in the lake. The excretion rates for the rotifers appear to be about two thirds of the total zooplankton excretion, even though the computed rotifer mean biomass is about one-third of the total zooplankton biomass.