GENETIC fine structure analysis of the rosy (ry) cistron in Drosophila melanogaster has shown that the rosy mutants can be resolved by crossing over into separable sites whose arrangement is consistent with a linear order (CHOVNICK, SCHALET, KERNAGHAN and TALSMA 1962; CHOVNICK, SCHALET, KERNAGHAN and KRAUSS 1964). Additional information concerning the rosy cistron and the region of chromosome 3 adjacent to it has been obtained by the analysis and utilization of rosy mutants associated with chromosomal changes. Such studies lead to the following generalizations: (1 ) In this region there is only one cistron concerned with xanthine dehydrogenase activity. (2) Mutations limited to the rosy cistron and selected on the basis of a mutant eye color phenotype are viable under routine culture conditions. (3) Lethal effects found with certain rosy mutants have proven to involve lesions outside of the rosy cistron and are associated with adjacent genetic units which are functionally and spatially distinct from the rosy cistron. A brief report of some of these results appeared earlier