Cell-free extracts of zooxanthellae (Symbiodinium sp.) from the hard coral Acropora formosa contained two acid phosphatases that were resolved by affinity chromatography on concanavalin-A-Sepharose. The enzymes had similar properties. with the exception that phosphatase P-1 hydrolysed polyphosphate and pyrophosphate, whereas phosphatase P-2 had no activity towards either. The high activity of phosphatase P-1 with polyphosphate implies that the physiological role of this enzyme may be the mobilization of this phosphate storage compound. The physiological substrate of phosphatase P-2 is unknown. but the most likely role of this enzyme is the hydrolysis of phosphate esters exterior to the plasmalemma, before uptake of the released morganic phosphate by the algal transport system. Cultured zooxanthellae (S. kawagutii) contained phosphatase P-2 only; the significance of this difference is unknown. The activities of P-1 and P-2 were always high in freshly isolated zooxanthellae. and both activities were repressed after incubation in phosphate-supplemented media. The implication is therefore that the algae in the coral-zooxanthellae symbiosis may be phosphate limited.