Alternating peaks of photosynthetic activity (P-phase) and nitrogen fixing activity (N-phase) were shown in Plectonema boryanum grown under diazotrophic condition in continuous light. In N-phase photosystem I activity increased by 35-45% and oxygen evolution decreased by 70-80% which was partially restored to 45-55% in the presence of 2,6-dichloro-p-benzoquinone. Thermoluminescence glow curve in P-phase consists of a major band at 25 degrees C which was shifted to 10 degrees C in N-phase. Both these bands are sensitive to DCMU and tetranitromethane. Flash excitation studies with major thermoluminescence band in P and N-phase showed oscillations with same periodicity of four having maxima at 2nd and 6th flashes. On the basis of these observations it is suggested that photosynthetic electron transport under diazotrophic growth condition is regulated by the redox levels of the secondary quinone acceptor QB.