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 EcoRl 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.