1. Accumulation of glycogen up to a constant amount per cell was observed during the postexponential phase of growth, in the presence of an excess of a utilizable carbon source. Cell multiplication was reproducibly controlled by growth of the organism in a nitrogen-limiting medium under photoautotrophic conditions (presence of light, air plus CO2). 2. Temporary starvation, i.e. by removal of light or by the addition to an illuminated culture of DCMU, 3-(3',4'-dichlorophenyl)-1,1'-dimethylurea, a specific inhibitor of photosystem II, lead to a mobilization of glycogen in the cell. Furthermore, Anacystis nidulans, having accumulated glycogen by virtue of preculture under nitrogen-limiting conditions, will resume cell division when the culture medium is complemented with a nitrogen source. The ability of the organism to use glycogen as an endogenous carbon source for growth was observed by addition of a nitrogen source to nitrogen-starving cells and simultaneous removal of CO2. 3. During the period of constant amount of glycogen per cell the reserve polysaccharide was subject to turnover as demonstrated with a pulse chase-labelling technique. The demonstration of a turnover--for the first time with a bacterial species--indicated a strict balance in the relative rate of synthesis and degradation.