The transformable cyanobacterium, Synechococcus sp. PCC7942, was used to study the genetics of resistance to the herbicide diuron. In wild-type cells, diuron binds to one of the core proteins, called D1, of photosystem II reaction centres. This binding prevents the transfer of electrons from QA, the primary quinone acceptor, to QB, which is necessary to create the charge separation that drives ATP synthesis. A single amino acid substitution in the D1 protein reduces diuron binding and confers herbicide resistance to reaction centres containing the substituted D1 protein. In Synechococcus 7942, the D1 protein is encoded by three functional genes called psbAI, psbAII and psbAIII. By selectively altering one member at a time of the three-member psbA gene family, we have demonstrated that diuron-resistant alleles are dominant to diuron-sensitive alleles. The relative abundance of the different psbA gene transcripts is correlated with the fraction of diuron-resistant reaction centres and with the degree of diuron resistance. Growth in sublethal diuron selectively increases the steady-state levels of transcripts of genes (psbA and psbD) encoding the core proteins of photosystem II. We have also found that turnover of the D1 protein can be uncoupled from electron transport through photosystem II.