Photosynthetic activities of submerged seaweeds before and after drying were measured in eighteen species collected from different vertical positions from a Pacific shore at Shimoda City in Japan. The seaweeds were desiccated until the cellular water levels equilibrated with the air in the chamber under various humidities at 10 °C. Relative water contents of all species drastically decreased as the cellular water potentials were reduced from 0 to −14MPa and gradually decreased in the range from −14 to −158MPa. Porphyra dentata, which is an upper intertidal species, recovered photosynthetic activity from the lowermost water potential of −158MPa, while the plants collected from the lower intertidal and subtidal zones reduced their photosynthetic activities after desiccation to comparatively higher water potentials from 0 to −14MPa. Other species collected from the middle intertidal zone reduced their photosynthetic activities following desiccation to water potentials in the range from −14 to −158MPa. It is concluded that the cellular abilities of desiccation tolerance in intertidal seaweeds varied between the species examined and correlated with their vertical position within the tidal zone.