The pineal of the facultative, cave-dwelling fish, Chologaster agassizi, was examined electron microscopically. Two cell types, photoreceptor and supportive cells, were identified in the pineal epithelium. The photoreceptor cells had well developed outer segments and contained Golgi bodies which were surrounded by both clear and dense-cored vesicles. Vesicle-crowned rods were frequently seen in various regions of the cell. The supportive cells also contained Golgi bodies from which both clear and dense-cored vesicles appeared to originate. In addition, these cells were characterized by peculiar arrangements of the smooth endoplasmic reticulum and the presence of pigment granules. Large quantities of glycogen were observed in both cell types. Small, unmyelinated nerve fibers were seen coursing throughout the pineal epithelium. Terminals filled with pleomorphic, clear vesicles and dense-cored vesicles were present in the vicinity of these nerve fibers. Similar vesicle-filled terminals were observed in close association with the supportive cells. The results of this study indicate that the pineal in this light-deprived species is a metabolically active organ capable of photoreception. Specializations of the organelles in the pineal cells were similar to those observed in other vertebrates living in environments of low light levels.