AbstractThe ultrastructure of spermatozoa from the cauda epididymidis and vas deferens ofOctodon degus— a Chilean hystricomorph rodent—is presented. The head of spermatozoa measured 7.7 μm long by 5.9 μm wide and the tail was 41 μm long. The head was flattened dorso‐ventrally and ovate in outline. The acrosome was the most distinctive feature ofO. degusspermatozoa. In a frontal view of the head, the rim of the acrosome surrounding the nucleus had the shape of an inverted U. The acrosomal region covering the plane of the flattened head exhibited dome‐shaped protrusions. Transverse or sagittal sections of acrosomal protrusions showed that the plasma membrane and outer acrosomal membrane were evaginated, while the inner acrosomal membrane followed the contour of the nucleus. The protrusions were not distributed at random and they were absent in the equatorial segment and in the rim of the acrosome.In frontal views, near the boundary between the acrosome and post‐acrosomal region, fine rods about 170 nm long ran obliquely on the caudal part of the equatorial segment. Behind the same boundary, the post‐acrosomal region showed a serrated border.Phosphotungstic acid treatment at pH 0.3 produced staining at the surface of the sperm as well as within a superficial layer of the marginal thickening of the acrosome and on the acrosomal