The northern snakehead Channa argus, native to China, Russia and Korea, is currently found widespread throughout Japan following its original introduction during the 1920s. A parasitological study of 10 snakeheads fished from the Fushinogawa River running through Yamaguchi City, Japan, detected 2-101 (average, 23.7) metacercariae per 100 g of trunk muscle from each fish. The trematode was identified as metacercariae of Posthodiplostomum sp. (Strigeidida: Diplostomidae) morphologically and characterized genetically based on the ribosomal RNA gene (rDNA). Phylogenetic trees were constructed on the basis of either the 18S, ITS or 28S region of rDNA to assess the relationship among members of the family Diplostomidae. The addition of genetic data from more diplostomid taxa, particularly Posthodiplostomum cuticola recorded from a variety of freshwater fish in Eurasia, would facilitate the precise identification of the present species.