Abstract
1. The X-ray mutability of wild-type isoalleles of the white-eye loci in Drosophila melanogaster has been determined.2. No difference in the mutability of the isoalleles was found. However, an appreciable difference in mutation rate of the two halves of the w loci was found.3. The bearing of these data on the problems of isoallelism and pseudoallelism are discussed.