A sperm defect causing infertility in two brothers is described. The malformation, named the "miniacrosome sperm defect," occurs in the total sperm population. It involves both the nuclei, which have a roundish apex, and the acrosomes, which are small, located in an apical position on the top of the nucleus, and sometimes ring-shaped. The midpiece and tail segments are normal, and the sperm are perfectly motile but fail to fertilize zona-free hamster ova in vitro. In spermatids, the absence of a microtubular manchette and reduction of the Golgi complex are evident.