Abstract / Introduction / Summary:
A juvenile of Nerocila japonica Schioedte and Meinert, 1881 was found parasitizing the ventral fin of a big-scaled redfin, Pseudaspius hakonensis (Günther, 1877) (Cypriniformes: Cyprinidae), in brackish waters of the Ten-no River on Sado Island, Niigata Prefecture, central Japan. The river flows into brackish Lake Kamo, which is connected to the southern Sea of Japan. The big-scaled redfin is an unusual cyprinid that can inhabit marine and brackish waters as well as fresh waters, and the fish caught most probably became infected by N. japonica at sea or in the lake and moved upstream in the river. The cymothoid is also considered euryhaline.