The ayu, Plecoglossus altivelis altivelis (Temminck and Schlegel, 1846), was examined for its copepod parasites from the lower reaches of the Kako River, Kakogawa, Hyogo Prefecture, Japan, in July 2017.
Two post-metamorphosis females of the lernaeid copepod Lernaea cyprinacea Linnaeus, 1758 were collected from two (25.0%) of the eight ayu examined.
Each infected ayu harbored one L. cyprinacea. This represents the first specimen-based record of L. cyprinacea from Hyogo Prefecture. The Kako River is the type locality of the ergasilid copepod Ergasilus plecoglossi Yamaguti, 1939, a gill parasite of ayu, which has not been reported from any locality since its original description. The copepod was not found in this study.