The genome of the cyanobacterium Anacystis nidulans R2 contains three genes (psbA) for the QB protein of photosystem II. This protein is essential for oxygenic photosynthetic electron tansport, and is the target for several herbicides which act by binding directly to the photosynthetic apparatus. Transcripts from the three Anacystis psbA genes are present in wild-type cells at different steady-state levels. The nucleotide sequences of two of the genes, psbAII and psbAIII, predict a protein having the same amino acid sequence which differs from that of the psbAI gene by 25 (out of 360) residues. Inactivation of each of the psbA genes in the Anacystis chromosome, singly or in pairs, shows that each of the genes is capable of producing sufficient functional QB protein to support normal photoautotrophic growth.