The mitochondria-rich. (MR) cells (also called chloride cells) in the gill epithelium of adult tilapia Oreochromis niloticus were investigated at the light and electron micro scopic levels. Thr•ee types of MR cells, the a, {3, and accessory cells are observed . In ad dition to numerous mitochond ria, a characteristic feature for the cytoplasm of the iden tified three types was the presence of a network of membranous tubules, the tubular system, connected to the basolateral plasma membrane. The f3 cells were found exclu sively in the interlamellar regions of the filaments. In addition to their well-developed tubular system, they contained numerous deeply stained elongated mitochond ria dis played closely packed cristae within a dense matrix. The cells were devoid of l.he api cal structures that were previousl y described in other fish. species. Small elongated cells were seen to be closely opposed to the lateral side of the f3 cells; they had a less developed tubular system and were considered to be accessory cells. The a cells were present along the lamellae. In contrast to the f3 cells, they exhibited a well-developed granular endoplasmic reticulum and the tubular system was lesser developed than in f3 cells. The possible role of these three types of MR cells in osmoregulation during ad ult life is discussed.