When carbon dioxide fixation was over 90% inhibited by CMU, nitrogen fixation remained unaffected in nitrogen-starved cells of Anabaena cylindrica. In normal cells under the same conditions nitrogen fixation was about 50% inhibited by CMU. These data suggest, first, that nitrogen fixation in this organism is independent of reducing potential generated by non-cyclic photo-electron transport and, secondly, that nitrogen fixation is stimulated by photosynthetically produced carbon skeletons to assimilate the fixed nitrogen. Although nitrogen fixation occurred to a limited extent in the dark, increasing light intensity stimulated nitrogen fixation both in the presence and absence of CMU. This suggests that light-generated ATP is required for nitrogen fixation in this alga. A ratio of pyruvate decarboxylation to nitrogen fixation of 3:1 has been established for A. cylindrica. This accords with the hypothesis that pyruvate acts as a hydrogen donor for nitrogen reduction and that provision of the required reductant is independent of photosynthesis in blue-green algae.