The intertidal snail Thais clavigera (Küster, 1858) has been believed to show a wide variability in shell morphology. However, the population in Tanabe Bay, on the west coast of the Kii Peninsula, is known to consist of only two forms, which are distinguishable by the shape of nodules on the shell. As the first step toward a better understanding of shell morphology variation, the existence of reproductive isolation between these two forms was tested in Tanabe Bay using allozyme analysis. The two forms were confirmed to be isolated genetically; there are significant differences between them in allele frequencies at the 6 analysed loci.