PhoX is an extracellular alkaline phosphatase that is widely found in cyanobacteria and plays an important role in the conversion of extracellular organophosphorus into soluble inorganic phosphorus. However, the gene has not yet been experimentally confirmed to exist in bloom-forming species. In this study, we identified a putative gene (GenBank accession no. ARI79942.1) in PCC7806 and overexpressed it in 21 (DE3). The expressed PhoX protein displayed phosphodiesterase and phosphomonoesterase activities. In contrast to other bacterial PhoX proteins, which are activated mainly by Ca, PhoX was most strongly activated by Mg, followed by Co, Ca, Zn and Mn, but it was inhibited by Ni. Sequence analysis showed that was highly conserved in the genus (DNA similarity?>?96% between species). expression responded significantly to different environmental phosphorus levels. When PCC7806 cells were cultured in phosphorus-deficient medium (BG11-P), expression reached its highest level at 2?h and then decreased to a low level at 4?h. Organophosphate induced the expression of ; its expression reached the highest level at 4?h and was maintained at a high level at 6?h. Our results confirmed a putative gene and demonstrated that the gene of is conserved.