delta-Crystallin of the embryonic duck lens was compared with that of the embryonic chicken lens with respect to polypeptide composition, synthesis, and messenger ribonucleic acid (mRNA) sequences. Labeling experiments with [35S]methionine revealed that the duck delta-crystallin is composed of minor amounts of polypeptides with molecular weights near 50000 (50K) and 49000 (49K) and much greater amounts of polypeptides with molecular weights near 48000 (48K) and 47000 (47K), as judged by sodium dodecyl sulfate-urea-polyacrylamide gel electrophoresis. All four sizes of polypeptides were synthesized in similar relative proportions as found in vivo in a rabbit reticulocytes lysate supplemented with delta-crystallin mRNA isolated from the embryonic duck lens. Synthesis of the 48K and 47K delta-crystallin polypeptides was differentially reduced in duck lenses cultured in the presence of ouabain. This is similar to the differential reduction of synthesis of the lower molecular weight delta-crystallin peptides in embryonic chicken lenses demonstrated previously. R loops formed between duck or chicken delta-crystallin mRNA and a cloned chicken delta-crystallin cDNA and heteroduplexes formed between duck or chicken delta-crystallin mRNA and cloned chicken genomic DNAs containing delta-crystallin sequences showed that, except for the putative 5' leader sequence, the duck and chicken delta-crystallin mRNAs have extremely similar nucleotide sequences. These data indicate considerable conservation of delta-crystallin throughout the approximately 100 million years of divergence between ducks and chickens. The findings also suggest a possible relationship between the structure of delta-crystallin mRNA and the differential reduction in synthesis of the lower molecular weight delta-crystallin polypeptides in ouabain-treated lenses of ducks and chickens.