Previous studies have shown that mutation of cyanobacterial zwf genes leads to markedly impaired dark viability, however, it is not entirely clear whether this phenotype is due to a polar effect on other genes in the transcriptional unit, in particular opcA. In the cyanobacterium Synechocystis sp. PCC 6803 the zwf and opcA genes are widely separated in the genome and can be mutated without polar effects on each other. The zwf gene of Synechocystis sp. PCC 6803 was deleted and no glucose-6-phosphate dehydrogenase activity was detectable in the mutant. Growth of the mutant was similar to the wild-type in the light, but also viability was unimpaired in the dark. The result of this study indicates that previously reported reduced dark viability in cyanobacterial zwf mutants may actually have been a result of impaired opcA transcription.