The covalent structure of rat ribosomal protein S19 was deduced from the sequence of nucleotides in a recombinant cDNA and confirmed from the NH2-terminal amino acid sequence of the protein. Ribosomal protein S19 contains 144 amino acids (the NH2-terminal methionine is removed after translation of the mRNA) and has a molecular weight of 15,944. Hybridization of the cDNA to digests of nuclear DNA suggests that there are 15-18 copies of the S19 gene. The mRNA for the protein is about 640 nucleotides in length. Rat S19 is related to Saccharomyces cerevisiae S16A and to Halobacterium marismortui S12.