Several workers have reported that most of the ribosomal DNA genes (rDNA) of the yeast Saccharomyces cerevisiae are located on chromosome I. More recently, data indicating that the yeast rDNA genes are located on chromosome XII has been presented. In this report, we present additional evidence indicating that most of the yeast rDNA genes are not on chromosome I. Starting from a diploid yeast strain, we isolated ten strains which were monosomic (2n-1) for chromosome I. We found that each of these ten strains contained two copies of the rDNA-containing chromosome. In addition, we show that the earlier evidence indicating that the yeast rDNA genes were on chromosome I cannot be explained by a difference in the yeast strains which were used in the different experiments.