The isoenzymatic pattern of glucose 6-phosphate dehydrogenase (abbreviation G6PD) from Ascaris suum has been studied by vertical polyacrylamide gel (PAGE) and horizontal starch gel electrophoresis. After polyacrylamide gel electrophoresis, two stained zones could be identified. One corresponded to tetrazolium oxidase activity; since this zone was stained even in the absence of glucose 6-phosphate (G6P), non-specific staining could be detected. In the other zone of activity, seven regularly-spaced bands were identified by staining in the presence of G6P and NADP as substrates. By using starch gel electrophoresis, different electrophoretic patterns for G6PD have been observed in the muscular sac, intestine and reproductive system from A. suum. The existence of three different alleles of G6PD in the same individual suggests the existence of at least two genes for this enzyme.