Abstract Azotobacter vinelandii ATCC 12837 when incubated on Burk agar containing 0.2 mg glucose ml −1 accumulated poly-β-hydroxybutyrate (PHB) prior to cyst formation. Addition of low levels of NH 4Cl, urea or adenine caused an increase in cyst formation, but PHB accumulation was reduced. Hypoxanthine and inosine also stimulated cyst formation on this medium although PHB accumulation was not reduced. When cultivated on Burk agar containing 0.2 mg β-hydroxybutyrate (BHB) ml −1, A. vinelandii cells did not accumulate large amounts of PHB prior to cyst formation. Addition of hypoxanthine, inosine, cAMP, AMP or adenine stimulated cyst formation on this medium. Again, PHB accumulation was not affected. It is concluded that cyst formation by Azotobacter does not require the accumulation of more than minimal amounts of PHB, and that the ratio and types of available carbon and nitrogen play a distinct role in encystment. In nature, Azotobacter probably encounters only small amounts of available carbon, but the environment also contains low concentrations of available nitrogen. It would seem that numerous cysts can be formed, without a requirement for hyper-accumulation of PHB, under these conditions.