Respiratory oxygen uptake was not suppressed by the inhibitor of the quinol oxidation site of cytochrome b/f complex 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), but by farred illumination in Synechocystis PCC 6803 cells grown photo-autotrophically. Ht is proposed that cytochrome b/f complex is not involved in the respiratory electron transport.