Catalytic conditions of the hydrolysis of phosphorus compounds have been examined in some eutrophic lakes as well as in axenic cultures of blue-green algae. The heat labile part of filtered samples of lakewater was revealing low, but distinctively pH-dependent activities of phosphomonoesterases (PME, E.C.3.1.3), when p-nitrophenylphosphate was used as a non-specific substrate. In 3 different lake biotops circadian fluctuations of PME were found to be corresponding with the content of dissolved phosphorus compounds and the counts of saprophytic bacteria. In vertical profiles of Lake Pluss activities of PME were subjected to considerable annual fluctuations. Here rather the kinetic parameters (Michaelis-Menten-constants) than the mere activities appeared to be correlated with the environmental concentrations of phosphate. These results corresponded in large part with some physiological regulations of phosphomonoesterase activities which could be observed in cultures of blue-green algae in relation to some defined environmental factors. An axenic strain of Anacystis nidulans was able to utilize different hydrolyzable phosphorus compounds up to a certain degree. In both the culture filtrates and the disintegrated cells the activities and the kinetic parameters (km; Vmax) were tested. The efficiency of dephosphorylation estimated by this means increased, when cells were grown in culture media limited by the source of P, or when growth had been retarded by other environmental factors.