A 23-kilobase EcoRI DNA fragment coding for the BALB/c immunoglobulin gamma 2a chain was cloned from mouse embryo DNA in the cosmid pJC74, and a nucleotide sequence of 1904 bases was determined for the entire constant region (CH1, CH2, and CH3), the three intervening sequences (IVS 1, IVS 2, and IVS 3) and the 5' and 3' flanking sequences. When the gamma 2a chain nucleotide sequence was compared with the gamma 2b chain nucleotide sequence, the percent homology of corresponding segments (excluding deletion and insertion) was 82% for the 5' flanking sequence, 87% for CH1, 84% for IVS 1, 96% for the hinge, 95% for IVS 2, 94.6% for CH2, 86% for IVS 3, 74% for CH3, 89% for the 3' untranslated region, and 92% for the 3' flanking region. These findings show that different domains of gamma 2a and gamma 2b have independent rates of evolution and that some of the noncoding segments of the gene are more conserved than are adjacent coding segments. Hypotheses on the possible role of IVS is gene evolution and expression are discussed.