The ultrastructure of spermatozoa in Hynobius leechii was examined with transmission electron microscope and compared with those of other urodeles and anurans. The perforatorium and the tail show most of the common feature of urodeles. However, there were several ultrastructural characteristics in an acrosome, nucleus, neck, axial rod and mitochondria related to the nucleus. The acrosome was trifoliate in transverse sections and the perforatorium consisted of two different concentric parts with a fine sharp point in a subacrosomal lumen. The nucleus consisted of two different regions of chromatin area and nuclear ridge. The nuclear ridge was composed of several bundles of five to ten minute tubular subunits of 19 nm diameter in this species, while in higher urodeles it was well developed in multi layers. The protoplasmic bead was separated by a cytoplasmic canal except the connection with only the distal portion of the nucleus. The neck was a short cylinder and contained pericentriolar material with transverse striations. In Hynobius the ring was not elongated to the tail and the mitochondria were distributed only in the protoplasmic bead around the nucleus as in Cryptobranchus, while in higher groups of urodeles it elongates to the length of the middle piece and the mitochondria follow the ring. The ring elongation may be related to the distribution of mitochondria. Hynobiidae and Cryptobranchidae are closely related based on structure of neck, the axial rod and location of mitochondria, although they are different in the composition of the axial rod.