Cyclic AMP was found at high concentrations in the cyanobacteria Anabaena cylindrica and Spirulina platensis but it was almost undetectable in Synechocystis PCC6803. Adenylate cyclase activity was detected in Spirulina and Anabaena but not in Synechocystis. There was a good correlation between the concentration of cellular cAMP and the activity of the enzyme.