A set of four clones containing the two adult beta-globin genes of the "single" type mouse C57BL/10 (genotype Hbbs/Hbbs) were isolated from a library of cloned restriction fragments. The two genes, designated beta s and beta t, were physically mapped onto a 32 kb segment of the chromosome carried by the four clones. Beta s and beta t form a stable heteroduplex 1850 bp long, indicating that they are intact and conserved at this level of resolution throughout their length, including their intervening sequences. The beta s gene allelic with the beta dmaj gene of the BALB/c mouse (genotype Hbbd/Hbbd). These two alleles, as well as their surrounding sequences, are highly conserved. In contrast, heteroduplexes of beta t with its BALB/c allele, beta dmin, revealed three extensive but localized rearrangements. One region of non-homology falls within the large intervening sequence, IVS2. To the 5' side of the beta/beta dmin gene position two unequal substitutions were observed; each results in the net insertion of about 1000 bp into the Hbbd chromosome. The beta/b dmin gene position is bracketed by a 1450 bp inverted repeat. One of the 1000 bp substitutions maps within this inverted repeat.