Phosphate-repressible alkaline phosphatases were found in axenic cultures of marine algae, especially Chrysophyceae and Bacillariophyceae. Enzyme synthesis started when the algae became P-deficient and stopped if phosphate was restored to the medium. Maximal enzyme activity was usually above pH9, but significant activity was also present at ordinary sea water pH. The phosphatases enabled algae to split glucose-6-phosphate; the phosphate was quickly assimilated but the glucose moiety remained in the medium. Algae took P from adenosine monophosphate and a -glycerophosphate at the same rate as from glucose-6-phosphate. The phosphatases appear to be firmly bound near the cell surface. They may enable deficient algae to regenerate phosphate from soluble organic P compounds present in natural sea water.