The fine structure of the mature plasmodia of Intoshia variabili, found in the kalyptorhynch flatworm Macrorhynchus crocea, was studied with transmission electron microscopy. As the plasmodium matures it forms long finger-like extensions inside the host. These extensions eventually take the place of the genital system of the host turbellarian. The mature plasmodium is separated from the host tissues by a continuous membrane. The entire plasmodium is surrounded by modified host cells, which are thin and overlapping. These cells also have a rather homogenous cytoplasm with limited endoplasmic reticulum. The surface of the plasmodium is mostly smooth and has numerous small extensions. Within the cytoplasm of the plasmodium, different stages of development of both males and females could be observed. The cytoplasm also contains numerous small nuclei with distinct nucleoli. Scattered throughout the cytoplasm, in addition to nuclei, are mitochondria, various granules, multivesiculate bodies with lamellar structures, and a large number of small vesicles. The nature of the formation of the plasmodium is discussed.