The status of the Kumamoto oyster Crassostrea sikamea in its native Japan is uncertain because of a lack of information about its abundance and distribution and a suggestion that C. sikamea and the Pacific oyster C. gigas hybridize in the northern Ariake Sea. Furthermore, broodstock populations on the United States Pacific coast have been hybridized with C. gigas in the past and may suffer inbreeding depression from multiple generations of hatchery-propagation. As a result, Japanese conservationists and United States oyster growers share an interest in the status of this species in the wild. We collected wild oysters from three sites in Saga Prefecture located in the northern portion of the Ariake Sea, Kyushu, Japan, in September 2006 and used molecular methods (species-specific PCR of the mitochondrial COI gene and PCR-RFLP of the nuclear ribosomal ITS1 gene) to assign 628 sampled oysters to one of three species found in this region. C. sikamea proved to be the dominant organism on artificial hard substrates, comprising 91% of the oysters sampled and typed. Many individuals confirmed as C. sikamea by diagnostic DNA markers had C. gigas-like phenotypes, such as striped shells. Crassostrea ariakensis was present (8% of typed oysters) but only at the lowest intertidal levels, and C. gigas was rare (1%) at these sites. We found no evidence of hybridization between any of the species and were unable to repeat a previous study, which suggested hybridization between C. sikamea and C. gigas based on sharing of a calmodulin allele. We conducted gamete compatibility tests among all combinations of Japanese (Ariake Sea) and United States C. sikamea and C. gigas broodstocks and found strong one-way gamete incompatibility (male C. sikamea × female C. gigas) between species of Japanese stocks, supporting the molecular diagnosis of C. sikamea. However, this one-way incompatibility was less evident in United States stocks, indicating lower barriers to potential hybridization in commercially cultured stocks.