In order to investigate the effect of salinity on growth and osmoregulation of the sweetfish, Plecoglossus altivelis, the fish (post-larva, juvenile, adult) reared in seawater were directly transferred into the experimental rearing water of the different salinities (post-larva : $0 extperthousand$, $5 extperthousand$, $10 extperthousand$, $15 extperthousand$, $20 extperthousand$, $30 extperthousand$. juvenile : $0 extperthousand$, $10textperthousand$, $20 extperthousand$, $30 extperthousand$. adult : $0 extperthousand$, $15 extperthousand$, $35 extperthousand$, respectively). Fish samples were taken in time courses after transfer for blood and body fluid collection. The changes of sodium, potassium concentration and osmolality of the body fluid and plasma, muscle water content, gonadosomatic index (GSI) and hepatosomatic index (HSI) in the experimental fish were observed in time courses. Growth rate of the post-larva in $5~25 extperthousand$ were faster than in $0 extperthousand$ (P<0.05). Survival rate was very low in $0 extperthousand$ (P<0.05). No fish survived more than 50 days after transferred into $0 extperthousand$. The concentration of sodium, potassium and osmolality in body fluid of the juvenile sweetfish were slightly decreased at 0.25 day after the beginning of the maintainable level of sodium, potassium and osmolality were $36.4pm6.2 mM/ell, 47.5pm0.7 mM/ell, 366.3pm19.1 mOsm/kg, respectively. Except potassium concetration, fish in $0 extperthousand$ and $10 extperthousand$ displayed the pattern of hypoosmotic regulation. Growth rate and survival rate of juveniles of $30 extperthousand$ are significantly low than other groups (P＜0.05). The concentration of sodium, potassium and osmolality in plasma of adult sweetfish were slightly decreased at 1 day after the beginning of the maintainable level of sodium, potassium and osmolality were $159.0pm2.5 mM/ell, 5.8pm0.9 mM/ell, 338.1$pm13.3 mOsm/kg respectively. Compared with these values, fish in $0 extperthousand$ and $15 extperthousand$ displayed hypoosmotic regultion. Growth and survival rate of adult in $35 extperthousand$ are significantly low than other experimental groups. GSI was the highest in $15 extperthousand$ after 60 days. HSI was the lowest in $35 extperthousand$. With decreased salinities, oxygen consumption was found to be decreased in the other experimental groups compared to the more than $30 extperthousand$.