ISHII(1931) described a nematode from the caudal fin of Japanese crucian carp, Carassius auratus (LINNe), under the name of Filaria carassii and stated that the gravid female of the nematode had three minute sharp protuberances around the mouth. Later, YAMAGUTI (1961) referred this species to the genus Philometra, but FURUYAMA (1934) and IVASHKIN et al. (1971) regarded it as the synonym of Philometroides sanguinea (RUDOLPHI, 1819) RASHEED, 1963. We found the eleven gravid females of ISHII's species in the caudal fin of nine of the 76 crucian carp which were obtained from a game fishing pond at Ichikawa city in Chiba prefecture in March, 1975, and examined for the nematode. The infected nine fish were 18-25 cm in body length. Five specimens of the eleven gravid females were examined morphologically. Possessing numerous bosess on the cuticular surface of their bodies, ISHII's species had to be certainly combined with the genus Philometroides and had to be distinguished from P. sanguinea in which the presence of the pharyngeal teeth was never described by any writer, though the sizes of gravid females, eggs and larvae were the same each other. We suggested from the combination of this species that the two characters of the genus Philometroides, that was, “Head with papillae” and “Oesophageal teeth absent”, had to be eliminated from the generic diagnosis proposed by RASHEED (1963).Philometroides carassii (ISHII, 1931) n. comb. (Figs. 1-4): Body filiform, tapered anteriorly and posteriorly with bluntly rounded end, 40-45 mm long by 0.8-1.1 mm wide when fixed in 5% formalin. Cuticle with numerous bosses irregularly scattered. Cephalic and caudal papillae absent. Three cuticularrized pharyngeal teeth present around the mouth. Pharynx swollen just under the mouth, 3.0-3.5 mm long by 0.11-0.15 mm wide at the swelling. Nerve ring 0.25-0.28 mm from the head end. Intestine 0.24-0.37 mm wide, ended blindly near the tail end. Anus and vulva absent. Uterus opposed. Anterior ovary 1.7-2.5 mm long by 0.05-0.08 mm wide and posterior ovary 1.2-2.0 mm long by 0.03-0.04 mm wide. Eggs spherical, thin-shelled, 26-40μ in diameter. Larvae filiform with rounded head and shrply pointed tail, 0.34-0.41mm long by 13-18μ wide. Types: Deposited in the National Science Museum, Tokyo. Cat. no. NSMT-As-1428.