Morphological, genetic and ecological analyses were performed on the Japanese chiton Acanthopleura japonica. Two morphological forms were identified. Spots were not observed on the shell plates of A. japonica collected from rocky shores around northern Japan (Form A). Two large black to dark-brown spots were observed in each shell plate, although sometimes not in the head plate, in populations from the Inland Sea, in the southwestern part of Japan (Form B). It is easy to discriminate Form A

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... Form B solely on the basis the pattern of the shell surface in both young and old specimens, despite the old ones being somewhat eroded. The 705 bp nucleotide sequence of mitochondrial cytochrome oxidase subunit I (COI) region of Form A was 7.66% different from that of Form B. Formspecific restriction fragment length polymorphism (RFLP) patterns were consistently observed when PCR-amplified COI region was digested with restricted enzyme FokI. These two haplotypes co-occurred on some shores in central Honshu. Form A tended to be abundant in the seaward quadrants and co-occurred with the barnacle Tetraclita japonica, whereas Form B was abundant on the landward quadrants and co-occurred with a population of the rock oyster Saccostrea kegaki at Nabeta Bay. There was a tendency for Form A to attach to the side of a rock facing the sea, whereas Form B were distributed on the same rock facing the shore, suggesting that the microhabitat of Form A and Form B may be different. Although Form A and Form B hybridize artificially, there is a possibility that these two haplotypes are reproductively isolated. We consider Form A and Form B to be in a sibling relationship.