Infusorigens of Dicyema typoides, and infusoriforms of D. typoides, D. aegira, and an undescribed Dicyema sp. were studied with the electron microscope. The infusorigen axial cell has a hyaline appearance and contains autophagic vacuoles. Spermatogenic cells are also hyaline, although they are more electron-dense than the axial cell. Spermatozoa are small and tailless. Mitochondria in infusorigen cells have few, short, tubular cristae. In infusoriforms, the tubular cristae are longer and more numerous, especially in the urn cells. Annulate lamellae are present in oocytes. Glycogen occurs in the infusorigen cells, but does not accumulate. Ciliated peripheral cells of mature infusoriform larvae are hyaline and start to degenerate while the infusoriform is within the host's renal sacs. Glycogen is abundant in peripheral cells of immature infusoriform larvae, but is less abundant in mature ones. Microvilluslike structures are present on the ciliated external cells. Apical cells contain little cytoplasm in mature larvae; in immature stages these cells contain small amounts of glycogen, and the ground substance is denser than when mature. Anterior medial bands, composed of deeply staining, granular material, appear to be formed within the enveloping cells of D. aegira. Typical cilia and ruffles (reported earlier on vermiform stages, Ridley, 1968), extend from the ventral internal cells into the urn cavity. Dorsal internal cells contain an abundant endoplasmic reticulum. Glycogen, which is especially abundant in dorsal internal cells of immature forms, is in the form of alpha rosettes which are smaller than those in external cells. Eosinophilic granules within the capsule cells contain an inner matrix in infusoriforms of mature Dicyema sp. No inner matrix was observed in eosinophilic granules of immature D. typoides. Urn cells contain many secretion granules and dictyosomes. Endoplasmic reticulum is more conspicuous in urn cells of immature infusoriforms than in those of mature stages. It is suggested that eosinophilic granules within the capsule cells might perform some sort of lytic function, possibly serving in the release of the urn cells from the infusoriform. The secretory granules within the urn cells may liquefy and form a cyst, with the eosinophilic granules of the capsule cells functioning in the liquefaction process. In the dicyemid mesozoans, infusorigens develop from infusorigen mother cells found within the axial cells of rhombogens. The infusorigen usually is considered to be an hermaphroditic individual which produces male and female gametes, the union of which results in the infusoriform embryo. The infusoriform apparently is the dissemination stage; it leaves the cephalopod host via the urine and its fate is unknown. References pertinent to the development and morphology of the infusorigen and infusoriform stages can be found in Lameere (1918), Nouvel (1947), McConnaughey (1951), and Short and Damian (1966). Bresciani and Fenchel (1967) have described certain aspects of the fine structure of the infusoriforms of Dicyema clausianum (from Octopus vulgaris) collected from Naples, and D. truncatum (from Rossia macrosoma) collected from the west