In cyanobacteria and other photosynthetic organisms, the mechanism of glucose-6-phosphate dehydrogenase (G6PDH) regulation has long been hypothesized to be a disulphide bond formation/reduction between -SH groups of two internal cysteine residues. In this study, four cysteine codons in the zwf gene of Synechocystis sp. PCC6803 were changed to serine codons, and then each mutation was separately transferred to the chromosome, yielding four cysteine-mutant strains designated ZWFCS101, ZWFCS187, ZWFCS265 and ZWFCS445. Thus, cysteine-mutant G6PDH was analysed in cyanobacteria. Cell-free extracts of the other three mutants were used as an enzyme solution. The mutant ZWFCS187, located close to the active site, completely lost all enzyme activity. To test whether the redox sensitivity of the mutant enzymes was changed, reduced dithiothreitol (DTTred) was added to the enzyme solution immediately prior to the assay. Three of the mutant G6PDHs were found to still be sensitive. This result is inconsistent with other reports where the cysteine-mutant G6PDH lost redox sensitivity. The results herein indicate that redox regulation is not the sole mechanism for the regulation of G6PDH activity, and the regulation most probably involves a sum of interactions at the cellular level, as well as additional contributing factors.