To understand the role of domestic birds in the 2009 H1N1 influenza A outbreak, a phylogenetic analysis of hemagglutinin, neuraminidase and matrix protein genes from human, avian and swine H1N1 viruses was carried out. Analysis of the H1 sequences revealed that the virus evolved most likely from American swine as well as intermixing between Asian swine and American domestic bird H1N1 viruses. Neuroaminidase and matrix protein analysis showed that the H1N1 2009 viruses were more closely related to the H1N1 isolates from Euro-Asiatic domestic birds and swine than wild birds. Domestic birds could act as intermediate hosts of H1N1 reassortants.