The sixth and seventh components of complement were found to be polymorphic and tightly linked in the laboratory opossum (Monodelphis domestica), as they are in eutherian mammals. In addition, strong evidence for linkage of the C6-C7 haplotype to the gene for glutamic pyruvate transaminase (GPT) was obtained for females but not for males. This result, combined with previous observations, established as a generality that recombination is severely reduced in females of this species by comparison with males. It also establishes synteny of C6-C7 and GPT in a marsupial species, as exists in mice. Because these loci are not syntenic in humans, the results imply that this synteny is ancestral to the separation of marsupials and eutherians and that it was broken relatively recently in the mammalian lineage leading to human beings. The newly described C6 and C7 polymorphisms provide additional power for developing a linkage map for M. domestica and for localizing genes that confer susceptibility to diseases for which this species is used as a model.