As seen with the electron microscope the integument of A. conchicola appears as a syncytium organized into a surface matrix and integumental cell bodies lying in the parenchyma. The matrix contains many vesicles and mitochondria. It is bounded on its distal surface by a dense layer organized into membrane-bound projections and on its proximal surface by a basal membrane and a fibrillar basal lamina. The basal lamina is interrupted by processes of the integumental cell bodies which wind through the parenchyma and muscle layer to connect with the matrix. Circular and longitudinal muscle fibers and muscle cell bodies are also found beneath the matrix. The integument of the ventral sucker is distinguished from the general surface by a thinner matrix, a thicker layer of muscle fibers running in 3 planes, a greater abundance of mitochondria in the matrix and muscle cell processes, and an additional cell type which is probably secretory in function. Recent studies have shown the ultrastructure of the integument to be basically similar in several digenetic trematodes (Bils and Martin, 1966; Burton, 1966; Lee, 1966; Erasmus, 1967; Threadgold, 1967, 1968; Morris and Threadgold, 1968; Smith et al., 1969), monogenoids (Lyons, 1970), and cestodes (Lee, 1966; Lumsden, 1966; Morseth, 1966; Bogitsh, 1968; Rifkin et al., 1970). These have revealed the integument to be a syncytium consisting of a surface layer joined by cytoplasmic extensions to cell bodies situated in the underlying parenchyma which contain the integumental nuclei. Stafford (1896) and Stunkard (1917) described the integument of aspidogastrid trematodes as observed with the light microscope, but apparently electron microscopy has not been applied to integumental structure and its bearing on host-parasite interactions in this group of parasitic flatworms. The study reported in this paper was designed to relieve that deficiency, using Aspidogaster conchicola von Baer, 1827, as the subject. MATERIALS AND METHODS Unionid collections were made from several localities in the Red River drainage in southern Oklahoma within the environs of the University of Oklahoma Biological Station, Willis, Oklahoma. Four species of mussels were examined for A. conchicola: Anodonta grandis, Tritogonia verrucosa, Quadrula quadrula, and Proptera purpurata. Worms were rinsed from the pericardial cavity into tap water, blotted dry, and either diced or fixed whole for 90 min in 2% glutaraldehyde in 0.1 M cacodylate buffer, pH 7.3, plus 5% sucrose; followed by Received for publication 20 May 1970. postfixation in 2% osmium tetroxide in 0.2 M s-collidine buffer (Bennett and Luft, 1959), pH 7.44, for 3 hr. After fixation and postfixation all tissues were rinsed in the appropriate buffer, dehydrated in ascending concentrations of ethanol and propylene oxide, and embedded in epon (Luft, 1961).