A study of the growth response of Lake Kinneret plankton to added phosphate and nitrate indicated that phosphate is the primary limiting nutrient. The residence time of soluble phosphate in lake water was measured by 32P-labelling experiments. Relatively long residence times of soluble phosphate were measured in winter and spring, during the Peridinium bloom. Two metabolic inhibitors, DCMU and CCCP, had similar effect on residence time during the Peridinium bloom and a significantly different effect after the bloom. These observations are shown to indicate that during the spring bloom, most of the phosphorus cycles between the lake water and the alga Peridinium cinctum fa westii, a dinoflagellate which causes heavy blooms and constitutes a nuisance to water users. The results show the importance of the intracellular phosphorus pools for algal growth. © 1979.