HE activity of xanthine dehydrogenase (XDH) in Drosophila melanogaster is affected by mutations at three loci: ma4 at locus 64ion the X chromosome, ry at locus 52 ion the third chromosome, and lxd at locus 33 ? on the third chromosome. See GLASSMAN (1965) for a discussion of these loci. The ma4 and the ry mutants are deficient in XDH while lzd flies have 20 to 25% of the XDH activity of wild type. To date, there has been no direct evidence that any of these loci is a structural gene for XDH. This paper presents data on electrophoretic variants of XDH due to changes at the ry+ locus.