ABSTRACT: Ultrastructural studies were undertaken of the host-parasite interface between Khawia sinensis Hsü, 1935 and Cyprinus carpio L. Pathology induced in the gut included loss of microvilli, compression of enterocytes and shedding of host material into the gut lumen. An interface layer occurred between the gut wall and the neck and posterior scolex of the parasite. A cellular response, which was evoked either by the worm or the damage induced, comprised eosinophils, macrophages and lymphocytes. Such leucocytes were also found in close association with the worm tegument.