Annual studies of kinetics of alkaline phosphatase (APA) activity and phosphorus availability for microplankton in the photic zone of an eutrophic lake are reported. The total APA activity of microplankton varied strongly. Vmax was highest during summer P depletion, and in autumn and winter total APA activity was low. The total APA specific activity of the microplankton was also highest (average 3.55 pmole PO4 (3-) ng ATP(-1) min(-1)) when ambient orthophosphate concentrations were very low. Both Vmax and specific APA activity were not dependent on the biomass of microplankton; they were strongly affected by P available for microplankton. A differential filtration technique was used for separation of microplankton into two size classes, i.e., algal, larger than 3μm, and bacterial fraction with size 0.2-3.0μm. The algal size fraction had lower specific APA activity (average 1.224 pmole PO4 (3-) ng ATP(-1) min(-1)) and higher KM values (38.8μmole × liter(-1)) than microorganisms which were smaller than 3μm (2.011 pmole PO4 (3-) ng ATP(-1) min(-1) and 25.4μmole liter(-1), respectively). The KM values of free, dissolved APA (36.8μmole liter(-1)) indicated that free APA was probably released by algae. Phytoplankton were major APA activity producers in the photic zone of the lake from March to November, and their activity constituted, on the average, 48.6% of the total APA activity in the water. Bacteria were the dominant APA activity producers in winter (41.3-44.9%); however, during other periods they contributed significantly (average 21.7%) to total APA activity. When surplus constituted less than 10% of particulate P in seston, phytoplankton produced high specific APA activity, and when surplus P was higher than 15%, the specific APA activity of phytoplankton size fraction rapidly decreased. APA of the bacterial size fraction of the seston was not affected by P concentrations. Orthophosphate was a competitive inhibitor of APA produced by microorganisms of the size fraction larger than 3.0μm, and increasing concentrations of inorganic phosphate caused an increase in KM values. The hypothetical metabolic-coupling between phytoplankton and bacterioplankton in the phosphorus cycle in conjunction with carbon metabolism in the lake is discussed.