The cyanobacteriaAnabaena torulosa andAnabaena L-31 require sodium (Na+) for growth on N2 but not in the presence of NH4 +. Na+-starved cultures show relatively little or no nitrogenase activity although they differentiate normal heterocysts. Nitrogenase activity appears rapidly on addition of Na+ to Na+-starved cultures. The time course of appearance of activity after addition of Na+ suggests that Na+ is involved in activation of the existing enzyme rather than in its de novo synthesis.