AbstractThe chloride cells of the lamprey,Lampetra japonica, have been studied by electron microscopy. The most characteristic feature of the chloride cells in lamprey is that the cytoplasm is filled with a large number of mitochondria and numerous agranular cytoplasmic tubules arranged as bundles or as branched reticular networks. These tubules are frequently continuous with the plasma membrane. They are also characterized by a coating on the cisternal surfaces with a dense material which appears ridge‐like at high magnification. These ridges are oriented spirally along the cisternal surface of the tubule. They are regularly spaced at an interval of about 160 Å and have a pitch of about 45°. The functional significance of the cisternal coat is unknown. Sometimes, intracisternal tubules are observed to be contained within the lumina of the agranular cytoplasmic tubules. These intracisternal tubules are considered to be formed by an intracisternal deflection of only one leaflet of the unit membrane of the agranular cytoplasmic tubules. Membrane fusion is observed to occur between closely associated agranular cytoplasmic tubu