We have determined the nucleotide sequence of a cloned beta-crystallin cDNA (pM beta Cr1) derived from the 5- to 10-day-old mouse lens and compared its deduced amino acid sequence with the amino acid sequences of the principal beta-crystallin polypeptide (beta Bp) (Driessen, H. P. C., Herbrink, P., Bloemendal, H., and de Jong, W. W. (1980) Exp. Eye Res. 31, 213-216) and a gamma-crystallin polypeptide (gamma II) (Croft, L. R. (1972) Biochem. J. 128, 961-970) of the bovine lens. A previous comparison indicated that bovine beta Bp and gamma II contain internally homologous sequences of amino acids and are evolutionarily related, suggesting an intragenic duplication of a common ancestral gene (Driessen, H. P. C., Herbrink, P., Bleomendal, H., and de Jong, W. W. (1980) Exp. Eye Res. 31, 213-216). When the amino acids were aligned, we calculated a 43% homology between the murine and bovine beta-crystallin polypeptides and a 22% homology between the murine beta-crystallin and bovine gamma-crystallin polypeptides. As for bovine beta Bp and gamma II, there is a striking homology between the amino and carboxyl halves of the murine beta-crystallin polypeptide. The internally homologous amino acids have been preferentially conserved among the three crystallin chains, a fact consistent with the possibility that these lens proteins arose from a common precursor gene that internally duplicated.