ABSTRACT: Recent advances in studies on osmoregulation in early life stages of fish are reviewed with special reference to extrabranchial chloride cells. Numerous chloride cells are present in the yolk-sac membrane of Mozambique tilapia embryos and larvae adapted to fresh water (FW) and seawater (SW). Chloride cells in SW often form multicellular complexes together with adjacent accessory cells, whereas chloride cells exist individually in FW. Chloride testing and X-ray microanalysis have shown that the SW-type, chloride cell complexes have the definitive function of chloride secretion. According to in vivo sequential observations on chloride cells in the yolk-sac membrane of tilapia, single FW-type chloride cells are transformed into multicellular SW-type cells in response to SW transfer, suggesting plasticity in the ion-transporting functions of chloride cells. A unique in vitro experimental model, named a ‘yolk-ball’ incubation system, has been established recently, in which the yolk sac is separated from the embryonic body and subjected to in vitro incubation. In the yolk balls prepared from FW tilapia embryos, chloride cells form multicellular complexes after SW transfer, indicating that chloride cells are equipped with an autonomous mechanism of functional differentiation that is independent of embryonic endocrine and nerve systems. The yolk-ball incubation system definitely serves as an excellent experimental model for further studies on chloride cell differentiation and functions. As chloride cells are commonly observed in the yolk-sac membrane and other body surfaces of fish embryos and larvae, it could be generalized that extrabranchial chloride cells play central roles in osmoregulation until gill chloride cells become functional.