Histological changes in the wall of the copulatory sac of normal adult female thorny skates, Raja radiata, were compared with those naturally infected with Trichodina oviducti. The parasites were associated with extensive defoliation of the copulatory epithelium and in some instances had penetrated the submucosa resulting in petechiae. An excessive exudate that appeared at the vent was made up of mucus, sloughed cells and parasites. It is speculated that shedding of the copulatory epithelium may be due to a hyaluronidase-like enzyme. Furthermore, the restriction of T. oviducti to adult skates may be dependent on the presence of certain mucopolysaccharides that are very low or absent in immature skates.