A monoclonal antibody (mAb) to epitopes on mitochondria from turkey spermatozoa cross-reacted with Japanese quail spermatozoal mitochondria. However, the pattern of binding was different from that observed for turkey sperm. The ultrastructure of quail spermatozoa was examined to determine the reason for this difference in antibody binding pattern. Light microscopy, as well as scanning (SEM) and transmission (TEM) electron microscopy, were used to study the morphology of spermatozoa from Japanese quail. Japanese quail had a sauropsid type of sperm cell, which is typical of nonpasserine birds. The spermatozoa were vermiform in shape, with a maximum width of 0.6 microm and an overall length between 230 and 250 microm. An acrosome (3.7 to 4.5 microm), nucleus (20.8 to 23.8 microm), midpiece (160 to 170 microm), and tail (40 to 60 microm) were observed. The TEM showed an acrosomal cap surrounding a perforatorium that inserted into the nucleus at the posterior end. Only a distal centriole was observed, which gave rise to a central axoneme with a 9+2 microtubular structure. The axoneme was encased by a spiraled mitochondrial sheath in the midpiece region (64 to 74% of the overall length of the sperm), and mitochondria numbers were estimated to be greater than 1,400 per sperm. In contrast, turkey sperm contain short midpieces with only 20 to 30 mitochondria per sperm. Differences in binding patterns of the mAb to turkey mitochondria between quail and turkey sperm were due to the presence of mitochondria on the exceptionally long midpieces of quail sperm.