A survey of helminth and crustacean parasites of 62 fish species from Lake Biwa and its watershed revealed eight parasite species suspected of being introduced. All were recorded from introduced host species from continental Asia or North America, or artificially stocked hosts (eels). The northern snakehead, Channa argus, hosts the nematode Pingus sinensis and the copepod Lamproglena chinensis. A Holarctic nematode, Raphidascaris acus, and a circum-North Atlantic tapeworm, Bothriocephalus claviceps, were found in eels, Anguilla japonica; both represent new records for Japan. Two North American monogeneans, Haplocleidus furcatus and Actinocleidus sp. (similar to A. fusiformis), found on largemouth bass Micropterus salmoides, represent the first occurrences of their genera in Japan. Of two undescribed acanthocephalans that represent the first records of the subgenus Acanthosentis in Japan, the one from the bitterling Rhodeus ocellataus ocellatus may well have been introduced together with its host. Another acanthocephalan, Pseudorhadinorhynchus samegaiensis, was originally described from rainbow trout, Oncorhynchus mykiss, at a hatchery. Now known from several native fish species around Lake Biwa as well, its status as a Lake Biwa area endemic or an introduced species is unclear. At present, no introduced digenean is known from fishes in this watershed.