The experiments showed that the osmotic stress-induced inhibition of nitrogen fixation activity was mitigated to some extent by the introduction of mannitol into culture solution in Anabaena sp. strain 7120. It was shown that under dark condition or in the presence of photosynthetic inhibitors, such as DNP or CCCP, the beneficial effect of mannitol on nitrogen fixation activity was weakened or didrinished in Anabaena. Higher nitrogen fixation activity was observed in experiments with mannitol by adding O₂ together with Hi, but the activity was depressed by adding O₂ alone. When Anabaena cells submitted to osmotic stress were incubated in aerobic conidition (in air) the beneficial effect of mannitol was higher than that in anaerobic condition (in Ar or Nc. Beneficial effect of mannitol on nitrogen fixation activity of Anabaena was obviously strengthened by preincubation with 5% CO₂ or with 5% CO₂ plus 40% N₂, but it was abated by adding N₂ only. The mannitol-supported nitrogen fixation aCtivity in Anabaena was less evident in the presence of exogenous sucrose.