Glandular adhesive organs and other structures by which turbellarians attach themselves temporarily to surfaces have been studied by electron microscopy. Adhesive organs in representatives of the turbellarian orders Haplopharyngida, Macrostomida, Polycladida, Rhabdocoela, Proseriata, and Tricladida are composed of three cell types: two gland cell types and a modified epidermal cell type through which the necks of the glands project. One of the gland types is characterized by its dense membrane-bound, 0.2–0.7Μm diameter, secretion granules and is called here theviscid gland on the basis of evidence that it secretes the adhesive substance by which the organs attach to surfaces. The other gland type is characterized by smaller (0.1Μm), less-dense, membrane-bound secretion granules and is called thereleasing gland on the basis of evidence that it secretes a substance by which adhering organs are released from surfaces. The modified epidermal cell is called theanchor cell; it has a well-developed cell web and bears microvilli with fibrous cores surrounding the tips of the gland necks in a collar-like fashion. Adhesive organs that have the two gland types, viscid and releasing glands, are referred to here asduo-gland adhesive organs. Other turbellarians, including orders Nemertodermatida, Acoela, and Lecithoepitheliata, have adhesive glands or other adhesive structures with a morphology unlike that of duo-gland adhesive organs. Ultrastructural characters of the adhesive organs show that the Macrostomida and Haplopharyngida are related, and that the Polycladida, Rhabdocoela, Proseriata, and Tricladida are related, and that these two groups of orders share a common ancestor. The Nemertodermatida, Acoela, and Lecithoepitheliata, with morphologically different adhesive systems, are apparently derived separately from these orders.