During infection in mice Echinostoma caproni is attached to the mucosa of the small intestine with the ventral sucker (acetabulum). The morphology, histology and dynamics of attachment sites from primary infections were examined. The sites were highly characteristic microscopically, and consisted of a plug of grasped mucosa occupying the cavity of the ventral suckers. The mucosa in the area of the small intestine where the parasite resided showed marked villous atrophy and crypt hyperplasia. The cellular composition of the attachment sites did not appear significantly different from what was seen in other parts of the mucosa in the residential area, and the study did not reveal any specific cellular host response at the attachment site. After mechanical removal of the parasites from unfixed tissue in saline, the attachment sites gradually reduced in size and disappeared. It is suggested that the attachment sites are only temporary structures, formed by the mechanical grasp of the ventral sucker as the parasites move about in the residential area of the intestine.