Two lines of experimental evidence indicate that the repeating ribosomal DNA (rDNA) genes of the yeast Saccharomyces cerevisiae are located on chromosome XII. First, the rDNA genes are linked mitotically to genes that have been previously mapped to chromosome XII. Second, yeast strains that have two copies of the chromosome containing the rDNA genes in every strain examined also have two copies of chromosome XII; this is not true for the other yeast chromosomes. These data also establish that in mitosis most of the rDNA genes in yeast are not extrachromosomal.