Abstract The genetics of expression of the endogenous C3H AKR-type murine leukemia virus (MuLV) was followed serologically in crosses with C57BL/6 and NIH Swiss mice. The serologically defined phenotypes in backcrosses with NIH Swiss mice were shown to correlate with the segregation of proviral DNA. The segregation patterns in backcrosses of NIH Swiss and C57BL/6 mice are most consistent with a single AKR-type MuLV genome in C3H mice. Linkage analysis of this locus shows no linkage with the genes for esterases 1 or 3, glucose-6-phosphate dehydrogenase, the β-hemoglobin chain, H-2, or the agouti color locus. In NIH × (C57BL/6 × C3H) mice the linkage of the C3H-MuLV locus with the single C57BL/6-MuLV locus was evaluated. The results demonstrate that the MuLV gene of C3H is not linked to the C57BL/6-MuLV gene.