This paper describes the cloning of full length marsupial type I interferon (IFN) genes and their flanking regions using a genome walking approach and PCR primers based on previously isolated partial DNA sequences. We confirm that the two major classes of Tammar Wallaby type I IFN genes are homologous with the eutherian IFN-α and IFN-β gene families. The wallaby IFN genes share a number of conserved features with their eutherian counterparts, including codons for cysteines at equivalent positions, implying similar secondary structures for the encoded proteins, and promoter regions with conserved putative regulatory motifs. Moreover, the wallaby genes have AT-rich elements in their flanking sequence corresponding to the mRNA 3′-untranslated regions, also implying that, as in eutherian mammals, rapid mRNA degradation plays a role in regulating expression of these genes. The complex nature of the type I IFN gene families in viviparous mammals (eutherians and marsupials) may reflect their recruitment into nonimmunological processes and this concept is discussed.