Ultrastructural studies on the integument of digenetic trematodes were investigated by several workers (Senft et al., 1961; Bjorkman and Thorsell, 1964; Burton, l966; Erasmus, l968; Morris and Threadgold, 1967, l968).

Clonorchis sinensis is a common parasite of human beings in the Far East. But as far little has been written on ultrastructure of the integument of this parasite.

This study was carried out systematically to elucidate the ultrastructure of the integument of adult C. sinensis, and the results are summarized as follows:

1. The integument of C. sinensis was organized in two parts; the epidermis and the muscular layer.

2. The apical surface and basal surface of the epidermis were covered with typical plasma membrane and irregularly invaginated from the surface. Plasma membrane at perisuckers was extremely folded and appeared as if the epidermis might be divided with the layer.

3. The syncytial epidermis contained mitochondria and numerous secretory bodies, and which was connected by cytoplasmic processes to nucleated epidermal cells lying in the parenchyma.

4. The mitochondriae, the secretory bodies, the granular endoplasmic reticulum, the Golgi bodies and the free ribosomes were present in the perinuclear cytoplasm of epidermal cells, but no organelle was observed in processes of epidermal cell except some secretory bodies.

5. The fibrous layer consists of argentophile fiber and collagenous fiber, in which the circular muscle bundles and longitudinal muscle bundles were embedded. Bundles of muscle fiber are weIl developed at the perisucker, and mitochondriae in this portion contain many cristae as compared with any other parts of C. sinensis.