This study was carried out to compare the ultrastructure of fresh, capacitated and acrosome-reacted sperm. The sperm was treated with heparin for capacitation and calcium ionophore for acrosome reaction induction. Sperm samples were then prepared for ultrastructural studies and examined by transmission electron microscopy (TEM). Ultrastructural changes in plasma and acrosomal membranes, shape of the mitochondria and outer dense fibres, in capacitated and acrosome-reacted sperm were evident. The plasma membrane of fresh sperm was loosely fitted around the sperm head and the acrosomal membrane was closely opposed to the nucleus. The plasma and acrosomal membranes of the capacitated sperm were expanded, but disintegrated in the acrosome-reacted sperm. Mitochondria of fresh sperm appeared to be rounded in shape with plasma membrane closely opposed to it and the nine outer dense fibres were almost regular rounded in shape. However, in both capacitated and acrosome-reacted sperm, the mitochondria were almost regular and elongated in shape whilst the outer dense fibres were irregular in shape in the capacitated and acrosome-reacted sperm. There were no noticeable morphological changes found in the axonemal complexes in fresh, capacitated and acrosome-reacted sperm. Ultrastructural studies are able to provide detailed information on sequential events involving numerous physiological changes during fertilization.