Two pufferfish individuals from the Seto Inland Sea, having an appear-ance intermediate between Takifugu vermicularis and T.poecilonotus, were morphologically and genetically examined. Both individuals showed a pattern of body spination intermediate between those characteristic of the two species, although principal component analysis (PCA) of the morphological data placed both specimens within the range of T.vermicularis. Isozyme analysis showed replacements of major alleles between T.vermicularis and T.poecilonotus at ll loci, the totally heterozygotic condition of those alleles suggesting that both specimens were F1 hybrids. Other morphological and whole genetic characteristics also indicated that they were F1 hybrids of T.vermicularis and T.poecilonotus. The inclusion of both individuals within the morphological range of T.vermicularis by PCA may have been the result of coupling of their parent species.
