Mouse, chicken and Xenopus laevis homologues to rig (rat insulinoma gene) cDNA were isolated and their nucleotide sequences were determined. Each homologue encoded a 145-amino acid protein; the amino acid sequence remained invariant in the murine and avian genes, and there were only 6 amino acid substitutions in the salientian gene. The evolutionary rate calculated for rig mRNA was sufficiently low to be viewed as evidence that rig is vital to vertebrate species. Southern blot analysis indicated that haploid sets of the mammalian genomes contain several copies of rig or rig-related sequences, whereas there appeared to be only one copy in the amphibian and bird genomes. The possibility that rig belongs to the class of housekeeping genes is discussed.