Abstract 1. The electrical potential difference (PD) and net water flux across the everted eel intestinal sac were estimated with a same preparation. 2. The water flux was closely correlated with serosa-negativity of the PD in sea-water eels but not in fresh-water eels. Similar situations were found in the conger eel (marine) and goldfish (fresh water). 3. When Cl − concentration of bathing solution was diminished, the high rate of water flux in sea-water eels decreased to a low level of fresh-water eels. 4. Osmotic permeability was six times higher in sea-water eels than in fresh-water eels. 5. The mechanism of intestinal water transport is discussed in relation to the development of Cl − pump during sea-water adaptation of the eel.