Observations to determine the time-dependent geographical distributions of caesium isotopes in the surface water were carried out in the western North Pacific during the period from 1979 to 1988. After the Chernobyl accident, the enhanced observation was carried out to study the geographical distribution of the Chernobyl radioactivity. The Chernobyl derived 1 3 4 Cs was detected in the surface water at the marginal seas around Japan north of about latitude 30°N, which reflects the meridional distribution of the Chernobyl derived 1 3 4 Cs in the surface air over the western North Pacific.