The ultrastructure and histochemistry of the tegument and penetration glands of adult Amphilina foliacea from the body cavity and the tissues of the internal organs of Acipenser ruthenus and A. stellatus were studied. New data on the localization in the tissue, development and in encapsulation of the adult A. foliacea mostly in the liver of A. ruthenus were obtained. The well developed penetration glands are necessary for penetration into the tissue and for migration of A. foliacea into the body cavity of the hosts. The tegument of the adult A. foliacea is a syncytium with cytons deeply embedded into the parenchyma. The secretory activity of the tegument of worms has a protective function against the immune system of the host. Our results give further information about the phylogeny of Amphilinidea and confirm the view of the close phylogenetic relationship of Amphilinidea and Cestoidea.