Abstract
Two allelic auxotrophic mutants at a locus close to the bw locus (2-104.5) of Drosophila melanogaster are described. The mutants respond to dietary ribonucleosides (uridine, cytidine, adenosine, guanosine and inosine) but less well to bases or pyrimidine precursors. This phenotype is unique to these mutants. We suggest that the mutants are defective in phosphoribosyl pyrophosphate biosynthesis.