Alkaline phosphatase (AP) plays an important role in the regeneration of bioavailable phosphate from organic compounds and allows phytoplankton growing in low inorganic phosphate environments to acquire phosphorus. We report the isolation, cloning, and initial characterization of the first AP (ehap1) in the coccolithophore Emiliania huxleyi (Lohm.) Hay and Mohler. This novel AP is a major form of AP released from the cell surface into the medium at late exponential and stationary phase of P-limited batch cultures but has no significant sequence similarity to other known APs. The cDNA sequence encodes a protein of roughly 94 kDa, while the processed active EHAP1 released from the cell surface is roughly 75 kDa. This difference is due to the cleavage of the signal sequence at its N-terminus and perhaps some truncation at its C-terminus. In response to variations in phosphate levels, the expression of ehap1 was found to correlate well with cellular AP activity. The ehap1 transcript was induced 4 h after phosphate depletion, increasing 1000-fold within a day, and was repressed rapidly upon phosphate addition. These results provide the basis for developing specific probes to study the expression of AP, and thus phosphate stress, of field populations.