AbstractFour‐day oldDrosophila melanogasterhomozygous or hemizygous for the glucose‐6‐phosphate dehydrogenase mutant allele,Zw−, possess slightly less than 10% as much glucose‐6‐phosphate dehydrogenase activity in whole‐fly homogenates as their wild‐type,Zw+, counterparts. The reduction of glucose‐6‐phosphate dehydrogenase activity inZw−adults had no effect upon 6‐phosphogluconate dehydrogenase, phosphofructokinase, or malic enzyme activities. However, activities of the non‐oxidative pentose shunt enzymes were significantly greater inZw−adults than inZw+adults. Radiotracer procedures indicate that the oxidative phase of the pentose shunt is blocked inZw−adults.Zw−larvae grow less well on a defined minimal amino acid diet lacking fatty acids and whole nucleic acid than doZw+larvae, indicating thatZw−individuals have a lower biosynt