Summary Y bb− is an rDNA-deficient chromosome of Drosophila that has often been used in magnification experiments to induce high-frequency reversion of bobbed ( bb ) chromosomes. We observed previously that Y bb− causes ring chromosome loss even when the rings are bb + , suggesting that Y bb− induces magnifying sister chromatid exchanges in bb + rings. Here we show that the Y bb- chromosome causes low levels of bb magnification in bb + flies. We refer to the ability of Y bb− to bypass the rDNA deficiency requirement for magnification as ‘constitutive’ magnification. We have magnified the ribosomal genes on the Y bb− chromosome and analysed the revertant chromosomes using genetic and molecular methods. We find that magnified Y bb− chromosomes also exhibit constitutive magnifier activity. Molecular analysis shows that both type 1 and type 2 intron + ribosomal gene repeats are associated with magnified Y bb− chromosomes. Type 2 introns have been described previously in the rDNA of both X and Y chromosomes. However, type 1 intervening sequences are thought to be present only in X , but not Y , ribosomal genes. Some of the Y bb− type 1 insertions differ from those present in the rDNA of X chromosomes in that they contain an Eco Rl site, and some may be present in tandem arrays. The constitutive magnifier activity of Y bb− may reside either in the structurally unusual ribosomal gene intervening sequences associated with the chromosome, or in the locus on Y L that is required for magnification to occur.