The genes opcA and zwf are located close to each other in most of cyanobacterial strains and the mutations in the opcA gene were reported to lose most of G6PDH activity. One of the reasons suggested for this loss was the polar effect of the mutation on expression of the zwf gene in the same operon and the other was absence of the OpcA polypeptide necessary for the catalytic activity of G6PDH. Synechocystis sp. PCC 6803 exhibits a gene organisation in which opcA and zwf are far away from each other and is ideal for analysis of an opcA mutation alone. In this study, an opcA single mutant and an opcA-zwf double mutant were constructed and effects of the opcA mutation on G6PDH activity and dark viability were then investigated. Contrary to the previous observations, no negative effect of the mutation on G6PDH activity was found under the optimal substrate concentrations. However, when one of the substrates, G6P or NADP, was reduced gradually, G6PDH activity in the mutant cells decreased faster than the wild types. Our results indicated that an opcA mutation did not affect G6PDH activity severely when zwf and opcA were in different operons. Similarly, dark viability of the opcA single and the zwf-opcA double mutants did not exhibit meaningful difference from the wild type.