It has been established using light and electron microscopy that the anterior adhesive apparatus of the skin-parasitic monogeneanAcanthocotyle lobianchi consists of three lobes on each side of the head, which are accommodated in a sac with a single aperture. Attachment is achieved not by suction, as previously proposed, but by dilation of the aperture and protrusion of the lobes, the surfaces of which are supplied with an adhesive secretion. A possible fourth lobe, described by a previous worker, has been shown to lie outside the adhesive sac and appears to be a non-adhesive, retractable papilla, abundantly supplied with compound and single cilia. This papilla seems likely to be a compound sense organ of a complexity not previously encountered in monogeneans. The adhesive surface of adults and oncomiracidia is supplied with two kinds of secretion and the possible functions of these secretions are discussed.