The 14S messenger RNA (1300 to 1500 nucleotides) for the alpha A chain of alpha-crystallin of the mammalian lens is nearly three times larger than required to code for the polypeptide that contains 173 amino acids. As a means of accounting for this anomaly, a complementary DNA clone for the mouse alpha A-crystallin messenger RNA was constructed in pBR322 and sequenced. Derivation of the protein sequence from the nucleic acid sequence showed that mouse alpha A-crystallin is similar to that of other organisms. The messenger RNA contains 536 nucleotides located on the 3' side of the coding region, excluding the polyadenylate stretch. This 3' sequence does not encode any other crystallin and has multiple termination codons in the three possible reading frames.