Takifugu rubripes, T. chinensis, T. pseudommus and an unidentified species were morphologically and genetically studied. Morphology was compared using external morphology and the genetic analysis was conducted using mitochondrial DNA cytochrome oxidase I (COI) and cytochrome b (Cyt b). The morphological characteristics of T. rubripes, T. chinensis and T. pseudommus were confirmed, as reported previously. The unidentified species was confirmed to have the characteristics of T. rubripes and T. chinensis. The COI sequences of the four species were 99-100% similar; genetic distance was d=0.0000 and Cyt b genetic distance was d=0.0000-0.00834. Species in the phylogenetic tree belonging to the same group could not be classified genetically. In conclusion, the unidentified species was considered to be a hybrid between T. rubripes and T. chinensis.