An adult male captured at Nakatsu village, Wakayama Prefecture, was identified as an interspecific hybrid between the Japanese and Taiwan macaques. Electrophoretic analysis of diagnostic blood proteins (adenosine deaminase, ADA; NADH-dependent diaphorase, DIA; transferrin TF) strongly supported the occurrence of hybridization between native Japanese macaques and artificially introduced Taiwan macaques in the prefecture. The male had a Taiwanese-like mitochondrial DNA. This suggested that the hybrid resulted from a mating between an immigrant male of Japanese macaque and a group-member, female Taiwanese macaque in the northern area of the Arida River.