Glucose-6-phosphate dehydrogenase (G6PD) is a ubiquitous enzyme catalyzing the oxidation of D-glucose 6-phosphate to D-glucono-lactone 6-phosphate, in the first step of the pentose phosphate pathway. Based on the currently available structural information on Leuconostoc mesenteroides G6PD, it is believed that the enzyme only works as a homodimer. Here we show that both after non-denaturing and after denaturing electrophoretic separation (SDS-PAGE) and blotting L. mesenteroides G6PD retains its complete catalytic activity. In the two latter cases the molecular weight of the band corresponded to that of a G6PD monomer. Conversely, when the same technique was applied to G6PD from Saccharomyces cerevisiae, another fermentative organism, the monomer activity was not detectable after SDS-PAGE and blotting. The results are discussed in terms of molecular evolution of the oligomeric state in the various G6PD sources.