Using a novel technique designed to identify genes of Saccharomyces cerevisiae which carry introns, we have cloned two genes encoding ribosomal protein S28. Although the genes differ by 15 nucleotides within their coding regions, they are predicted to encode identical proteins of 145 amino acids. The predicted amino acid sequence of S28 contains significant homology to ribosomal protein S25 of Tetrahymena thermophila and to ribosomal protein S12 of several archaebacteria, suggesting a relationship to S12 of Escherichia coli. Dot matrix analysis confirmed that regions of S12, especially those implicated in the accuracy of translation, have been conserved in S28 of S. cerevisiae. Either RPS28A or RPS28B alone can support growth, but heterozygous disruption of both genes abolishes the ability to sporulate. Haploids harboring a disruption of both genes cannot survive without an intact gene on a plasmid. RPS28A maps to the right arm of chromosome VII and RPS28B to the right arm of chromosome XVI.