To examine the role of the retinoid X receptor (RXR) in the development of imposex in gastropods, we investigated the time course of expression of the RXR gene in various tissues (ctenidium, ovary or testis, digestive gland, penis-forming area or penis, and head ganglia) of female and male rock shells (Thais clavigera) exposed to triphenyltin (TPT) in a flow-through exposure system for 3 months. Accumulations of TPT in tissues were clearly observed in exposed individuals, whereas no accumulation of TPT was observed in the control groups. In females, 3-month exposure to TPT resulted in the development of imposex, and penis lengths in imposex-exhibiting females were significantly longer in small females (shell height <20 mm) than in large females (shell height > or = 20 mm). RXR gene expression in the ovary, penis-forming area or penis, and head ganglia of females exposed for 3 months was significantly higher than expression in control females, and the highest RXR gene expression was found in the penis-forming area or penis. Moreover, RXR gene expression in the penis-forming area or penis of each female exposed to TPT seemed to be associated with an increase in penis length. In males, the ratio of penis length to shell height was significantly larger in the exposed groups than in the controls. Although RXR gene expression in males exposed for 3 months was not significantly higher than expression in control males in any tissues, the highest gene expression was observed in the penis of exposed males. These results suggest that RXR plays an important role in the development of male genitalia (i.e., penis and vas deferens) in gastropods, although RXR might also have other physiological functions.