The Red-billed Leiothrix Leiothrix lutea has been introduced to, and become established in Japan, the Hawaiian Islands and certain areas of Europe. Unlike in its native range, introduced Red-billed Leiothrix populations have not been observed to be hosts of avian brood parasites. We found a parasite egg with three host eggs in a Red-billed Leiothrix nest on 14 July 2010 on Mt Tsukuba, central Japan. Despite its marked difference from the host eggs, the parasite egg was accepted and incubated by the l Red-billed Leiothrix parent until the nest was depredated before hatching. The morphology of the parasite egg suggested that it belonged to Oriental Cuckoo Cuculus optatus.