ABSTRACT The question of whether the Ybb - chromosome contains ribosomal genes has been examined by using Southern blot analysis and comparing rDNA hybridization patterns for X/X and X/Ybb - DNA. The results demonstrate that the Ybb - chromosome contains sequences that hybridize to an rDNA probe under stringent conditions. Differential hybridization of some of these sequences with DNAs corresponding to different regions of a complete ribosomal gene repeat provides evidence that some of the genes on the Ybb - chromosome are type 2 repeats. Because data obtained by other workers suggest that type 2 repeats are transcribed only to low levels, these repeats may be classed as "nonfunctional". A further finding is that the ribosomal genes on the Ybb - chromosome do not undergo multiple rounds of DNA replication during polytenization of X/Ybb - cells.