The trematode Hasstilesia ovis parasitic in sheep was studied by scanning electron microscopy. A single type of sensory papillae was found on the surface of oral and ventral suckers and around the genital pore. The whole surface of body, except the oral and ventral suckers and vicinity of genital pore, is covered with spines. The mostly multipointed and rarely singlepointed spines are of variable size and distribution on different parts of body. They irritate mechanically the intestinal mucosa of the definitive host and thus increase the pathogenic effect of the trematode.
