Macrocephalic sperm are defined as sperm with a head notably larger than the heads of a normal population of sperm for that species. This defect has been described in mammals, frequently in association with other abnormalities such as multiple tails, cytoplasmic droplets and abaxial implantation, but represents a rare defect in birds. Routine examination of emu and ostrich semen smears revealed an incidence, expressed as a percentage of total sperm defects, of 11–12% sperm with enlarged heads. In the emu, macrocephalic sperm were commonly associated with multiple tails, while in the ostrich they displayed what appeared to be a single, but visibly thicker, tail. A similar head:tail ratio of 1:2.3 for macrocephalic sperm was evident for both species, compared to the ratio for normal sperm of 1:3.9 in the emu and 1:3.7 in the ostrich. Ultrastructural features of the defect were similar in the two species. Both displayed a unitary midpiece housing a variable number of centriolar complexes (CC), each surrounded by a mitochondrial sheath incorporating a single row of mitochondria between adjacent CC. The nuclear base appeared flat or gently scalloped, and was widened to accommodate attachment sites for the supernumerary CC. Cytoplasmic droplets were frequently associated with the multi-centriolar midpiece of the ostrich. In the emu, the proximal parts of the multiple principal pieces often spiraled around each other but the tails were generally separated, free and of similar length. The thickened, apparently single tail seen in macrocephalic ostrich sperm by light microscopy was shown ultrastructurally to consist of multiple principal pieces collectively bound within the plasmalemma. Macrocephalic ostrich sperm therefore also possess multiple, but unseparated, flagella. The combination of enlarged head and multiple tails, termed the macrocephalic sperm head syndrome in man, may result from incomplete cytokinesis or abnormal centriolar duplication in the dividing spermatogenic cells.