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A case of headless or decapitated spermatozoa of an infertile man is reported. The 33-year-old patient showed almost normal spermiogram except for a high percentage (more than 90%) of headless spermatozoa. Both hypoosmotic swelling and zona-free hamster egg tests showed that most headless spermatozoa appeared to be normal in their function as tails. The ultrastructure of the ejaculated spermatozoa was observed using a surface replica method and by conventional ultrathin sectioning. The headless spermatozoa were almost normal in structure except for the absence of the head. Tailless heads were rarely observed. Varieties of ultrastructural abnormalities were observed in spermatozoa heads. Among them, abnormalities in the nuclear membrane covering the posterior pole of the nucleus were prominent. The implantation fossa and the basal plate were not formed. Instead, the nuclear membrane with numerous nuclear pores was found in this region. The decapitation seemed to take place between the region of the basal plate and the proximal centriole.