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The effect of different growth conditions on the glycogen and poly-/I-hydroxybutyrate (PHB) content of the cyanobacterium Spirulina maxima is described. Under photoautotrophic growth conditions without any nutrient limitation, S. maxima exhibited a glycogen content of between 7-1 and 10.7% of cell dry wt, whereas PHB was undetectable. When S. maxima was grown under mixotrophic conditions in the presence of acetate, the intracellular PHB concentration increased to more than 3 % of dry wt, while glycogen content remained within the range of 5 to 6% of cell dry wt. Nitrogen starvation favoured glycogen accumulation (up to 60 to 70% of dry wt), while the PHB content remained low (up to 0.7% of dry wt), even after prolonged nitrogen starvation. Inhibition of protein synthesis, induced by addition of azaserine, led to the accumulation of glycogen (up to 52 yo of cell dry wt) but did not stimulate PHB synthesis. Under phosphorus-limited growth conditions, glycogen and PHB accumulated (up to 23% and 1.2% of cell dry wt, respectively) only after the exhaustion of intracellular phosphorus reserves. Shifting the culture from low to high light irradiance induced a rapid accumulation of glycogen (up to 34% of cell dry wt after 9 h) but did not induce PHB synthesis. Results are discussed in terms of the metabolic significance of PHB synthesis in cyanobacteria, and suggest that this polymer acts exclusively as a disposal mechanism to eliminate excess reducing equivalents.