1. Phosphorus limitation was studied along the eutrophic, canyon-type Rimov reservoir (Czech Republic) during a spring phytopiankton bloom. Concentration of soluble reactive phosphorus (SRP), C:P molar ratio in seston, extracellular alkaline phosphatase activity (APA), and P limitation (bioassay) were used as indices for phosphorus deficiency in the phytopiankton. 2. SRP, C:P, APA, and P limitation indicated a moderate P deficiency in the downstream, but not upper, part of the reservoir. 3. Significant correlations between these parameters were found in the downstream part. Chlorophyll a concentration correlated with APA and P limitation in the upper part. 4. APA was significantly enhanced in the phosphorus-^deficient phytopiankton. However, APA was apparently not related to total biomass or spedes composition of the phytopiankton. 5. Generally, APA was closely correlated with pH in the reservoir. However, extracellular alkaline phosphatases, with a pH optimum above 9.0, were induced and active only during the phytopiankton bloom, whereas low background activity of extracellular phosphatases was found at low chlorophyll a concentrations (wirxter, dear-water phase).