Semen sample was collected from two captive adult Yangtze finless porpoises (Neophocaena phocaenoides asiaeorientalis) during physical examination. One individual was aged about 9 years with body length 143 cm (total length) and body weight 46.1 kg in 2003. The age of the other was unknown and its body length was 147 cm and body weight was 43 kg in 2004. Ultrastructure of their spermatozoa was examined using scanning and transmission electron microscope. The sperm concentration was 4.17 × 10⁹ spermatozoa per ml by the cytometer. The approximate dimensions of the spermatozoa were as follows: head length, 3.366 ± 0.140 μm (mean ± SE, n = 15); head width, 1.896 ± 0.099 μm (n = 15); and neck length, 1.004 ± 0.074 μm (n = 10). The tail included midpiece, principal piece and terminal piece. The length of the midpiece was 1.882 ± 0.077 μm (n = 9). There is no apparent boundary between the principal piece and the terminal piece, so the length of the principal piece and the terminal piece was 44.612 ± 3.485 μm (n = 5). Total length of the spermatozoa was 53.314 ± 4.580 μm (n = 10). The acrosome covered approximately 45.8% of the anterior portion of the head.