Ayu Plecoglossus altivelis is a native amphidromous fish in Japan and an important target for farming and recreational fishing. We have found high infection of hitherto unknown digenean metacercariae in wild ayu larvae and juveniles captured from the sea and a river in Wakayama Prefecture. The infection prevalence was 95% and 100% in larvae and juvenile ayu, respectively with variable intensities up to 566 metacercariae per fish. The metacercariae were located primarily around fins and inside

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... udal fins, but appeared to cause no serious pathological features to the fish. The phylogenetic analysis based on the internal transcribed spacer 2 and 28S rDNA sequences revealed its identity as a member of Bucephalinae (Bucephalidae) with no identical genetic information on the database. Morphological characteristics of the larval trematode resembled those of the genus Prosorhynchoides. Genetically identical metacercariae were also found from Scomber japonicus and Engraulis japonicus captured at the same location as P. altivelis with variable infection levels, which suggested that it was a marine species. Experimental infection challenge of the metacercariae to several marine fish via oral intubation failed to obtain adult specimen and thus the precise identification of the parasite remains unknown.