Heterocyst differentiation in the cyanobacterium Anabaena cylindrica is initiated by the removal of fixed nitrogen from the medium. These specialized cells occur singly at regular intervals within filaments of vegetative cells. Incubation of cultures for periods of up to 12 h immediately prior to or following removal of fixed nitrogen, at a light intensity (500 mi Einsteins cm-2 s-1) approximately 10-fold higher than that required for optimum growth, resulted in the differentiation of pairs of adjacent (double) heterocysts. The frequency of double heterocysts was proportional to the length of the period of high light intensity. During growth at normal light intensity approximately 5% of cell divisions were symmetrical, but this increased more than 3-fold during 10-h incubation at high light intensity. The frequency of dividing cells remained constant during this period, but increased rapidly on return to normal light. The frequency of double heterocysts was reduced if a period of incubation at normal light intensity was interposed between the 12-h period at high light intensity and transfer to nitrogen-free medium. A period of 8 h normal light was required to reduce the frequency of double heterocysts to control values, and this corresponded to the length of time needed for the frequency of symmetrical divisions to return to control levels following 12 h at high light intensity. We confirm that cell division in Anabaena cylindrica is asymmetrical and conclude that the presence of double heterocysts results from an increase in the symmetry of cell division during incubation at high light intensity. The results also support the finding of previous workers that a cell is only susceptible to differentiation during a short period following its formation. During the period of high light the rate of doubling of the absorbance of the culture at 750 mn increased from 24 h to approximately 10 h and decreased to more than 100 h on return to normal light. The very high rate could be explained by increases in the volume and granular content of cells during incubation at high light intensity and did not represent an equivalent increase in the rate of cell division.