The sperm cells in the ejaculate of a sterile Ayrshire bull were studied by light microscopy as well as scanning and transmission electron microscopy. The spermatozoa showed an almost total lack of tails, whereas the heads appeared morphologically normal. The cytoplasmic residue at the caudal end of the head contained numerous membrane structures as well as mitochondria and incomplete elements of the proximal centriole and striated column. The basic disturbance appears to be a hereditary defect in the spermiogenesis.