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Many species of the filamentous N₂‐fixing heterocyst‐forming Cyanobacteria of the genus Nostoc produce large amounts of extracellular polymeric substances (EPS), but hitherto no general model has been proposed of the factors that control their synthesis. Previously, we demonstrated a strong correlation between the presence of a glycocalyx (or EPS capsule) and diazotrophic growth in the genus Nostoc. When grown with nitrate, nude morphotypes lacking a glycocalyx were obtained for all the capsulated strains tested. CO₂ availability was pro‐posed as a key factor that controls the synthesis of the capsule. To test this hypothesis, Nostoc PCC 7936 was cultured diazotrophically (N₂) or with nitrate with different CO₂ supplies. By tuning the pH and the supply of CO₂, capsulated or nude mor‐photypes were obtained irrespective of the source of nitrogen. Exocellular polysaccharides were synthesized only when …