The stratified, eutrophic ůímov Reservoir (South Bohemia) receives a considerable input of phosphorus. However, phytoplankton development in the epilimnion has often been P-limited (<5 µg.1−1 of soluble reactive phosphorus) during spring and summer when algal maxima are observed. Alkaline phosphatase activity (APA) was measured in the epilimnion (0-2 m) using a fluorimetric method. The seasonal course of APA (range 6.7-726.7 nmol.1−1.h−1) roughly followed that of chlorophyll a (r2 = 0.45) in 1990. During algal blooms, a significant percentage of total APA was found in the 2.5-100 µ µm fraction containing most of the algae. Very similar relationships were found by a longitudinal sampling of the reservoir epilimnion during the spring phytoplankton bloom in 1991, despite high P concentrations in the reservoir's tributary. Hence, phytoplankton can be P-limited in the epilimnion, even in a eutrophic reservoir with a high input of P. This study shows that the phytoplankton community might adapt to phosphorus depletion by changing species composition and/or by increasing APA.