Inositol monophosphatase (IMPase; EC 3.1.3.25) acts at the last step in the inositol biosynthesis pathway by hydrolysing inositol-1-phosphate into inositol. In this study, an IMPase encoding gene, all2917 from Anabaena sp. PCC7120, was characterized. We found that All2917 exhibits a specific activity on inositol-1-phosphate, in a typical Mg(2+) -dependent, Li(+) -sensitive manner. The deletion of all2917 in Anabaena made the cells more sensitive to osmotic stress caused by sucrose or sorbitol, while its overexpression led to an increased resistance to such stress. Consistent with these phenotypes, the transcription of all2917 was significantly upregulated upon the sucrose-mediated osmotic stress. Phylogenic analysis using 134 IMPase homologues from 36 cyanobacterial strains shows that members of IMPase family form three major distinct clades, suggesting that multiple copies of IMPase family proteins have been maintained in Cyanobacteria during a long history of evolution, and they may play important roles in cyanobacterial physiology.