Cessation of growth with N2, NO3− or NH4+ as nitrogen source marked the beginning of akinete differentiation in Anabaena doliolum. The primary trigger of the differentiation process during diazotrophic growth conditions appeared to be the generation of a nitrogen starvation signal resulting from progressive loss of the activities of nitrogenase and glutamine synthetase. Persistence of a substantial level of oxygenic photosynthetic activity long after the cessation of diazotrophic growth and the initiation of akinete development suggests a role for oxygenic photosynthesis in the process of akinete development subsequent to its initiation. The loss of diazotrophy much earlier than that of oxygenic photosynthesis explains the observed increase in the C/N ratio occurring during initiation and the middle phase of akinete formation. Both the initiation of akinete development subsequent to cessation of diazotrophic growth and the rise in the C/N ratio during the early and middle stages of akinete formation appear to be regulated by a developmental mechanism similar to the one that functions in the control of nitrogenase and glutamine synthetase activities during diazotrophic cyanobacterial growth.