The ultrastructure of guinea spermatozoa was characterized by transmission and scanning electron microscopy. The average lengths and widths (measured at the widest points) of the segments were as follows: acrosome, 1.8 x .47 mu; nucleus, 12.8 x .49 mu; midpiece, 3.9 x .59 mu; flagellum, 59 x .52 mu. Thus, the guinea sperm cell is slightly smaller than the chicken or turkey spermatozoon. The ultrastructure of the guinea spermatozoon was similar to that of the chicken and turkey with two exceptions: 1) the mitochondria had cristae aligned parallel and oblique with respect to the outer limiting membrane and 2) there was no proximal centriole, the distal centriole projected into the implantation fossa with "arms" of the nonstriating connecting piece extending laterally from the centriole to the implantation fossa.