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The spermatozoa of the neotropical hystricomorph rodent Ctenomys maulinus have been examined cytochemically and under the transmission electron microscope. The head is flattened dorsoventrally. At the caudal end of the head there is a process oriented parallel to the tail. This process corresponds to a cylindrical extension of the nucleus, which constitutes a unique feature among mammals.

Spermatids and epididymal spermatozoa from wild burrowing hystricognate rodents were examined. Structural defects affected the acrosome, the nucleus and the tail, and were similar to those found in the developing germ cells and spermatozoa from domestic mammals and man. The acrosome vesiculation of epididymal spermatozoa from hystricognate rodents might not be necessarily an abnormality and may represent a step in the acrosome reaction. Major abnormalities, such as invagination of the acrosome granule, incorporation of lytic vesicles into the forming acrosome, detachment of the acrosome from the flat surfaces of the nucleus, invagination of the nuclear envelope, and the occurrence of multinucleated and multitailed elements may be caused by a variety of factors, but those related to seasonal reproduction (breeding season) should not be neglected.

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