The Ybb− chromosome has been previously shown to induce reduction of X chromosome ribosomal genes in Xbb / Ybb− or Xbb+ / Ybb− flies. These reduction events are presumed to arise as one of the two products of unequal sister chromatid exchanges, which result in both magnified and reduced products. Bobbed reduced chromosomes may also arise as products of other recombinative events such as intrachromatid deletions. In this report we use the Ybb− chromosome to reduce the number of ribosomal genes present on X chromosomes from two wild-type stocks under ‘non-magnifying' conditions. We then show that the bobbed reduced X chromosomes show no detectable difference in their Southern blot rDNA patterns when compared with the parental wild-type X chromosome. This indicates that reduction events do not preferentially delete certain repeat classes, and supports previous observations that the repeat types present in the D. melanogaster X chromosome nucleolus organizer are not significantly clustered. © 1984, Cambridge University Press. All rights reserved.