AbstractAnalyses of the fine structure of the posterior choroid plexus in the teleostLeuciscus rutilusand the determination of the presence and function of the enzymes acid and alkaline phosphatases, ATPase and glucose‐6‐phosphatase confirm similarities between these epithelial cells and thesaccus dorsalisand also with the epithelial cells of the choroid plexus found in mammals. The teleost plexus cells contain coated vesicles which are derived from the plasmalemma as well as from the Golgi complex. Moreover, they contain multivesicular bodies and Iysosomes. These organelles function in the absorption of substances from the cerebrospinal fluid and in the breakdown of these substances within the cells. The investigated enzymes play an important role in the secretion of electrolytes into the cerebrospinal fluid by active membrane transp