Batch culture experiments were performed to test the ability of Phaeocystis sp. to grow on organic phosphates as the only source of phosphorus.Of nine organic phosphates tested seven supported growth equally well as did inorganic phosphate Growth on cyclic-adenosine monophos-phate (c-AMP) was slow compared with inorganic phosphate, while myo-inositol hexaphosphate (phytin) was not used for growth by Phaeocystis. The relative affinity of Phaeocystis alkaline phosphatase for a number of organic phosphates, including those used in the growth tests, was determined by measuring the ability of the organic phosphates to inhibit the hydrolysis of a test substrate by the enzyme. The relative affinity was expressed by means of an inhibition constant (K1) values for c-AMP and phytin were relatively high, indicating low affinity of the enzyme for these substrates K1, values for the other phosphates tested were much lower indicating high affinity of the enzyme. Comparison of the results of the growth experiments with the Kt values indicated that the latter were suitable instruments for predicting the outcome of the growth experiments The results imply that in natural environments Phaeocystis can use enzyme-hydrolysable organic phosphates efficiently for growth.