An ultrastructural study on the mature spermatozoa of Clonorchis sinensis was carried out. For this study, the liver flukes were collected from the livers of rabbits and rats artificially infected with the metacercariae obtained from the fresh water fish, Pseudorasbora parva. Six-month old worms were used. The collected liver flukes were washed with 0.85 percent saline solution and then immediately moved to cold 2 percent glutaraldehyde buffered with 0.l M Millonig's phosphate buffer (pH 7.4). The materials were dissected into appropriate pieces in the fixative about 30 minutes after beginning of the fixation. Two hours later the materials containing the seminal receptacle were rinsed several times with the buffer and were secondarily fixed with cold, buffered 1 percent osmium tetroxide for 2 hours. The fully fixed tissue blocks were dehydrated in a series of graded concentrations of acetone and were embedded in Epon 812 mixture. Thin sections obtained from LKB-5 ultramicrotome were stained with uranyl acetate and Reynold's lead citrate. Observations of the sections were carried out with JEM-100CX II electron microscope. In general, the mature sperm was long thread-like form with a sickle-shaped head. According to the longitudinal sectioned view of the sperm tail, the nucleus seemed to be spirally coiled and run a little far along the tail. The acrosome was not observed. The cytoplasm of the tail was biflagellated as usual in trematodes. Unlike other platyhelminth spermatozoa, the sperm tail of Clonorchis sinensis showed the [9+2] pattern in the microtubular arrangement. The mitochondria with poorly developed cristae were observed throughout the middle piece. The middle piece of the tail showed dull ladder or triangular shapes with the two flagella at the bottom. But, the principal piece of the tail was slightly flattened cylindrical shape with two flagella within the cytoplasm. The end piece was uniflagellated. It was not clearly identified whether the end piece was subdivided into two by flagellum or the lengths of the two flagella were different. The glycogen granules were rich in the cytoplasm throughout the length of the spermatozoa. These granules might be the energy source for the movement of the spermatozoa.