Transmission electron microscopy was used to determine the organization and fine structure of the gastrodermis of two species of Gyrodactylus. The gastrodermis consists of a syncytial epithelium characterized by large widely spaced nuclei, two types of vesicular structures, mitochondria with tubular cristae, Golgi complexes, and scattered rough endoplasmic reticulum; the luminal surface possesses few widely spaced, branched lamellae projecting into the intestinal lumen. Organization of the gastrodermis in the Monogenoidea is discussed; the syncytial gastrodermis is considered to be a possible syn-apomorphic feature of the Gyrodactylidea. Two general patterns of nutrition occur within the Monogenoidea (see Halton & Jennings, 1965). In the subclass Polyonchoinea, member species usually feed on epidermal tissues and secretions of the host, and species of the subclasses Polystomatoinea and Oligonchoinea are generally sanguinivorous. These nutritional preferences appear to be related to gastrodermal structure and organization. In the Polystomatoinea and Oligonchoinea, the gut wall is comprised of two cell types, one of which is involved in digestion of blood (see Rohde, 1980, for review). In polyonchoinean species investigated to date, the gastro-dermis comprises a monolayer of cells consisting of a single cell type (Fournier, 1978; Halton & Stranock, 1976; Junchis, 1988). Apparently, the latter organization of the gastrodermis is symplesiomorphic for the Monogenoidea in that a similar state also occurs among rhabdocoel turbellarians (Holt & Mettrick, 1975) as well as among some digeneans and aspidobothreans (Dike, 1967; Hathaway, 1972; Rohde, 1971). The purpose of this paper is to report an apomorphic state of the gastrodermis within the derived polyonchoinean family Gyrodactylidae.