In order to investigate the osmoregulation capability of grey mullet, Mugil cephalus with the different salinities, juvenile fish $(13.6{pm}0.2;TL)$ stocked in seawater (SW) were abruptly transferred to each experimental group $0\%SW(0\%_{circ}),;25\%SW(7.7\%_{circ}),;50\%SW(16.1\%_{circ});and ;100\%SW(32.8\%_{circ})$ and reared for 60 days. Blood samples were taken by the time schedule after the transfer. Plasma $Na^{+},;K^{+},;Cl^{-}$ and osmolality, muscle water content, and the electron microscopical observations of chloride cells were analyzed and made by the time schedule. In $100\%SW$, the maintainable levels of plasma $Na^{+},;K^{+},;Cl^{-}$ and osmolality were $167.1{pm}7.7mM/l,;9.1{pm}2.1mM/l,;137.8{pm}5.6mM/l;and;351{pm}18;mOsm/kg$, respectively. These values were significantly changed at $6h~1;day$ after the beginning of the experiment with four different salinities. Fish from $0\%;and;25\%SW$ had lower osmolalities than those of fish from $50\%;and;100\%SW$, and showed the hyposmotic regulation pattern. At the end of the experiment (60 days after transfer), however, no significant difference was found in the concentrations of plasma $Na^{+},;K^{+};and;Cl^{-}$ among four experimental groups. Hematocrit was increased with salinity (P<0.01). After 10 days, fish from $0\%;and;25\%SW$ showed the hypertrophy, fusion and edema of epithelial layer in gill lamella. However, at the 15th day, epithelial layer in gill lamella was back to the normal status. On gill of fish from $0\%SW$, one apical pit held two or three chloride cells in common. Muscle water content was subsequently regulated to near the normal levels within 4 days, and there was no significant difference among four different salinities at the end of the experiment.