In order to assess the occurrence and the degree of phosphorus deficiency, the upper layer (0-2 m) of the eutrophic Římov Reservoir (Czechoslovakia) was sampled from March to October 1990 in one- to three-week intervals. Soluble reactive phosphorus and maximum possible orthophosphate (by Rigler's radiobioassay) concentrations, seston elemental stoichiometry (C/P; N/P), orthophosphate turnover time, and phosphorus deficiency index (= ratio of the primary production to the Vmax of the PO4 uptake) were estimated. Early in spring, during the spring clear-water period, and in the autumn, no indications of P-deficiency were found. Strong P-deficiency was detected in summer (June - September), moderate to strong deficiency also during the spring phytoplankton maximum. The concentration of SRP < 5 µg.1−1 indicates phosphorus deficiency in the Říimov reservoir.