Microtubules (MTs) of cells of Spirogyra sp. were depolymerized by treatment with amiprophos-methyl (APM) for 1 h and then reorganized in 0.30 M mannitol solution. The reorganized MTs after 1.5 h incubation showed an oblique/longitudinal orientation and then became transversely oriented as the incubation was prolonged. During this incubation, the osmotic pressure of cells was measured by the plasmolysis method. The cell osmotic pressure increased with time. The calculated turgor pressure at 1.5 h was 0.11 M (mannitol equivalent) and, at 13.5 h, 0.25 M. Similar changes in MT orientation and recovery of the turgor pressure were also observed in 0.30 M sorbitol solution. These results suggest that the MT orientation may be correlated with the turgor pressure. Among fresh water algae sensitive to a saline environment, this Spirogyra was the first species shown to have a turgor regulating mechanism, although the recovery of turgor pressure was incomplete. The recovery of turgor pressure in mannitol solutions was also observed without APM treatment.