Inactivation of the genes for the cyanobacterial phytochromes cph1 and cph2 in the unicellular cyanobacterium Synechocystis sp. strain PCC 6803 affected the growth of the cells under certain light conditions. Differences in growth were detected by recording growth curves and in competition experiments. Mutation of cph1 and cph2 resulted in different effects. The cph1⁻ mutant strains exhibited a reduced growth rate under far-red light (FRL), whereas the growth of the cph2⁻ mutant strains was inhibited by red light (RL). The growth rate of a cph1⁻/cph2⁻ double mutant was reduced under both RL and FRL. Furthermore, cph1⁻, cph2⁻ as well as double-mutant strains showed impaired growth under high-light (HL) conditions. Acclimation of the photosynthetic apparatus of the mutants to RL, FRL and HL, as determined by pigment analysis, was similar to that of the wild type.