ABSTRACTMultiple copies of the 18s and 28sribosomal RNA cistrons are present in both the Xand Y chromosomes of Drosophila melanogaster.Data are presented here that identify a locus, Rex,that causes exchange-like events between duplicated ribosomal complexes at the ends of an attached-XY chromosome. Rex:(1) is close to or in the basal heterochromatin of the Xchromosome; (2)is semidominant and (its effect) is temperature sensitive; (3) acts maternally; and (4) affects behavior of paternally derived attached-XY chromosomes shortly after fertilization. Though, at this point, the existence of Rexis known only from its effects on behavior of a particular compound chromosome, it presents intriguing possibilities for understanding regulation of chromosome behavior and organization of the ribosomal cistrons.