The analysis of the developmental pattern of the αA‐, αB‐, βB1‐, βB2‐, βB3‐, βA3/A1‐, and βs‐crystallin genes during fetal and postnatal development of the rat shows that the differential regulation of crystallin synthesis relies on differential gene shutdown rather than differential gene activation; that is, all crystallin gene are active during early development but turn off at different stages. The only two exceptions to this rule are the αB‐ and βs‐crystallin genes. The αB‐crystallin gene transcript becomes first detectable at 18 days of fetal development, while the βs‐crystallin gene appears to be active only in the postnatal period.We also determined the absolute numbers of the αA‐, αB‐, βB1‐, βB2‐, βB3‐, βA3/A1‐, βs‐, and γ‐crystallin gene transcripts present in the lens at various times after birth. Comparison of these RNA data with the published protein data shows that the αB‐ and βB2‐crystallin RNAs are relatively overrepresented, suggesting the possibility that these two RNA species are not used as efficiently as other crystallin mRNAs. Examination of the known (hamster) αB‐crystallin sequence and elucidation of the (rat) βB2‐crystallin sequence yielded no evidence for aberrant codon usage. These two RNAs have one sequence motif in common: they are the only crystallin mRNAs in which the translation initiation codon is preceded by CCACC.