We have isolated and characterized a cDNA encoding Drosophila melanogaster myo-inositol-1-phosphate synthase (INOS). The deduced Drosophila INOS protein is 50% identical to the Saccharomyces cerevisiae INO1 gene. The putative active site residues are well conserved in Drosophila INOS protein. Southern blot analysis shows that Drosophila INOS gene is a single copy gene. Northern blot analysis reveals that Drosophila INOS gene expresses a 2.0-kb transcript that is more abundant in the head than the body, suggesting that it may be involved in brain function. The recombinant Drosophila INOS protein was expressed in Escherichia coli and the purified protein has proved to have a myo-inositol-1-phosphate synthase activity.