The organization of the connective tissue of the polyclad turbellarian Discocelides langi has been studied using light microscopical, histochemical and electron microscopical techniques. Filamentous components are found in the subepidermal membrane and in the body, especially around muscle cells. The subepidermal membrane is complex, consisting of an outer limiting component and the membrane proper. The membrane proper is composed of filamentous lamellae. The filaments, probably belonging to the collagen family of proteins, form an orthogonally arranged grid system. Large amounts of ground substance are found in the interior of the body. Histochemical observations indicate that it probably consists of both neutral and acid polysaccharides in combination with protein components. On electron micrographs it presents itself as a finely granular to fibrillar net-like material of low density. A frequently occurring component is a prominent system of membranous strands, composed of polysaccharide-protein materials. They are in continuity with and similar to cellular mucous coats or glycocalyx components. The inherent difficulties in identification of connective tissue cells are discussed. In the peripheral parts of the animal, cells which are closely apposed, and reminiscent of planarian fixed parenchyma cells are found. In the interior parts cells similar to these are present but are embedded in ground substance. The cells often are large and branched and contain a cytoplasm with few organelles and of very low density on electron micrographs. Another type is strongly basophilic due to a high content of RNA. The fine structural morphology is very varied but is always characterized by the presence of a great number of ribosomes, either free or membranebound. This type possibly represents the formative cells responsible for synthesis of protein filaments and ground substance. The connective tissue system found is discussed in relation to other types of turbellarian connective tissue. Also the functional implications are commented on.