Previous studies indicate that genes from only one of the cell's nucleolus organizers undergo multiple rounds of DNA replication in polytene cells of Drosophila. This report presents evidence that this effect is mediated by a function that is associated with the ribosomal genes of the dominant or replicating X chromosome. This function can act in trans to result in replication of the ribosomal genes on the recessive X chromosome in flies that are bobbed for the dominant X chromosome. In these cases, ribosomal genes from both chromosomes undergo polytenization. Heterochromatic regions that flank the nucleolus organizer have little or no effect on nucleolar dominance. In addition, deletion of the compensatory response (cr(+)) locus does not affect the dominance, suggesting that ribosomal gene compensation and nucleolar dominance in polytene cells of Drosophila are separate genetic phenomena.