Adult Clonorchis sinensis from infected human cases may have black pigment in their parenchyme. In present paper, the nature of the pigment was investigated. The pigment distributed in cytoplasm of parenchymal cells of the worms. Histochemically the pigment was positive by periodic acid-Schiff, Ziehl-Neelsen, and lipofuscin stainings. The pigment particle contained many, various sized membrane structure as well as electron dense granules when observed by electron microscopy. Staining characteristics of the black pigment, together with electron microscopic findings, strongly indicated that the pigment was lipofuscin.