To investigate the DNA surrounding genes for immunoglobulin heavy chain constant (CH) regions, we have isolated two clones bearing a Cγ3 gene and two bearing a Cγ3 gene from a library of mouse embryo DNA fragments. The Cγ3 clones span 8.6 kilobase pairs (kb) on the 5′ side of the gene and 6.7 kb on its 3′ side, while the Cγ1 clones together span 13 kb of 5′ flanking sequence and 2.5 kb of 3′ flanking sequence. Restriction mapping of the Cγ3 gene indicates that intervening sequences divide the gene into segments of domain size, as in other CH genes. Hybridization of clone fragments to restriction digests of mouse DNA indicates that both the Cγ1 and Cγ3 genes probably occur as single copies in the genome. Moreover, the entire cloned sequences on the 5′ side of both genes appear to be unique in the genome, indicating that no large common sequences flank CH genes. Restriction data suggest that the Cγ3 gene is 37-40 kb 5′ to the Cγ1 gene.