While a null activity mutant allele of the structural gene for 6-phosphogluconate dehydrogenase in Drosophila melanogaster is lethal, a similar mutation for glucose-6-phosphate dehydrogenase is not. Double mutant combinations lacking both enzyme activities, obtained either by recombination or by mutagen treatment of a chromosome bearing the lethal allele, result in a restoration of viability. The indispensability of the pentose phosphate shunt in Drosophila appears to depend upon the specific position of the block within the pathway.