Abstract: The microstructure and ultrastructure of the spermatozoa of Japanese eel, Anguilla japonica, (n=10) artificially induced by extraneous hormone were studied. By means of light microscopy, two types (type I and type II) of spermic shapes could be observed. The spermic head of type I was almost round in shape, and similar to the shape of strawberry. The size of type I spermatozoa was smaller, with (2.57±0.62) μm of the long diameter of the karyon, (2.11±0.59) μm of the short diameter of the karyon, and (38.35±7.71) μm of the flagellum length. The spermic head shape of type II was similar with the shape of eyebrow or crescent. This size of type II spermatozoa was much bigger, with 7.66±1.09 μm of the long diameter of the karyon, (2.54±0.46) μm of the short diameter of the karyon, and (38.35±9.02) μm of the flagellum length. By means of transmission electron microscopy (TEM), different ultrastructures of two types of spermatozoa could be observed. For type I spermatozoa (round spermatozoa), there was no acrosome on the forefront of the spermic head. Implantation fossa was located in the middle of the spermic head bottom, and it is in the shape of a channel along the long axis of spermatozoa. The centriolar complex was inside implantation fossa. Two to three pieces of mitochondria were under karyon. The head of basal body was co lumniform in shape, and consisted of electron density substance. The end part of basal body was separated into 9 strips. Spermic flagellum prolongated from the sleeve, and the initial part of axoneme linked up the end part of basal body. The microtubule was electron dense. The axoneme formed a typical 9+2 pattern. Type I spermatozoa might be normal spermatozoa that reached to maturity. For type II spermatozoa (eyebrow-like spermatozoa), the karyon was curving in shape, and there was a globoid object in the curving area. There were mitochondria and centriolar complex inside the globoid object. The axoneme formed a 9+0 pattern. Type II spermatozoa might be immature or abnormally developed spermatozoa.