Abstract VSP is a transmembrane protein whose cytoplasmic region shows significant similarity to phosphatase and tensin homolog deleted on chromosome 10 (PTEN). Notably, VSP exhibits a unique ability to transduce electrical signals into phosphoinositide turnover by coupling a transmembrane voltage sensor domain to the PTEN-like phosphoinositide phosphatase domain. Moreover, VSP gene is known to be widely conserved among deuterostome genomes, though the function of VSP in vivo remains largely unknown. In the present study, the expression pattern of ascidian VSP( Ci-VSP) was examined in embryos and juveniles of a marine invertebrate chordate, Ciona intestinalis. RT-PCR showed that Ci-VSP is expressed at the larval stage and that expression persists in juveniles. Whole mount in situ hybridization showed that Ci-VSP is expressed in cells of the stomach, intestine and blood cells of 2- to 3-week-old juveniles. Moreover, double staining blood cells from 2-month-old adults with Ci-VSP and Ci-PTEN probes showed that Ci-VSP-positive cells are a distinct population, separate from cells expressing Ci-PTEN. These findings suggest that in addition to its previously suggested roles in testis or sperm, Ci-VSP plays a key role in voltage-induced signal transduction in cells of the digestive system and blood.