A boar with decapitated spermatozoa was examined. The growth rate and libido were normal. The right testis was in the scrotum, but the left one was intra-abdominal. Ejaculated spermatozoa and tissue specimen from both testes were observed by light and electron microscopy. Only tailless heads and headless tails of spermatozoa were observed in the ejaculate. The ratio of heads to tails was about 1:4. Motility of the headless tails was about 25%. The proximal extremity in the tail was occupied by the proximal centriole. The basal plate could not be found in the head or in the tail. Pathogenesis of decapitated spermatozoa in the present case was analysed by investigating spermiogenesis in the scrotal testis. Since the pair of centrioles failed to approach the nucleus, mechanical connection between the proximal centriole and the nucleus did not seem to be established. In addition, the basal plate was not formed on the nuclear membrane. Since the ratio of tailless heads to headless tails was also 1:4 in the testis, it was concluded that the heads had already detached the tails in the testis.