Three phosphate-regulated proteins in the coccolithophorid Emiliania huxleyi were detected by the biotinylation of cell-surface proteins. Two of these phosphate-regulated proteins have reduced denatured molecular weights near 110 000 Da (118 078 and 110 541, respectively), while the third, and most abundant, is 69 087 Da. Induction of the three proteins and the common marker of phosphate stress, alkaline phosphatase activity, occur in the presence of <0.25 μM inorganic phosphate in batch culture. Phosphate-regulated proteins and enzyme activity differed among E. huxleyi strains. Alkaline phosphatase is an enzyme commonly induced by phytoplankton in response to phosphate stress in order for cells to scavenge inorganic phosphate from organic sources. In E. huxleyi, this enzyme activity and the phosphate-regulated proteins are rapidly lost when phosphate is added back to phosphate-stressed cultures. This contrasts with the slower loss of alkaline phosphatase activity in the dinoflagellate Prorocentrum minimum. The presence of the three phosphate-regulated proteins and enzyme activity appear to differ somewhat among E. huxleyi strains. Based on these differences between strains, kinetic data, growth experiments and enzyme activities, the 69 087 Da protein may be a phosphatase with a high specificity for 5′-nucleotides.