Abstract: The classification of Carassius has not been well established due to its great variability and wide distribution. Usually, Carassius is identified as three species: C. carassius, C. cuvieri and C. auratus, the latter including several subspecies, such as goldfish. Out of these subspecies, C. auratus gibelio have recently been thought of as a valid species of Carassius. In this study we collected the 5′end 651 bp segments of the mitochondrial cytochrome c oxidase I (COI) gene from 128 specimens, including C. carassius, C. cuvieri, C. auratus auratus, C. auratus gibelio and C. auratus langsdorfii. All three species of Carassius (C. carassius, C. cuvieri, C. auratus) were found to be valid, meanwhile genetic differentiation between the Eurasian C. auratus and Japanese C. auratus has reached a high level. However, several haplotypes were shared between C. auratus auratus and C. auratus gibelio. Consequently, C. auratus gibelio should be regarded as a subspecies of C. auratus rather than a valid species. Moreover, because both diploids and triploids exist in C. auratus auratus and C. auratus gibelio, ploidy level should not be used as criteria for the classification of species or subspecies in Carassius.