This study identified a novel influenza A virus, designated as H18N11, in a flat-faced fruit bat (Artibeus planirostris) from Peru. Serologic studies indicated that several Peruvian bat species were infected by this virus. Phylogenetic analyses revealed that New World bats harbor more influenza virus genetic diversity in some gene segments than all other mammalian and avian species combined, suggesting a long-standing host-virus association. Structural and functional analyses of the hemagglutinin and neuraminidase proteins indicated that sialic acid is not a ligand for virus attachment nor a substrate for release, suggesting a unique mode of influenza A virus attachment and activation of membrane fusion for entry into host cells. These findings indicate that bats constitute a potentially important and likely ancient reservoir for a diverse pool of influenza viruses.This study identified a novel influenza A virus, designated as H18N11, in a flat-faced fruit bat (Artibeus planirostris) from Peru. Serologic studies indicated that several Peruvian bat species were infected by this virus. Phylogenetic analyses revealed that New World bats harbor more influenza virus genetic diversity in some gene segments than all other mammalian and avian species combined, suggesting a long-standing host-virus association. Structural and functional analyses of the hemagglutinin and neuraminidase proteins indicated that sialic acid is not a ligand for virus attachment nor a substrate for release, suggesting a unique mode of influenza A virus attachment and activation of membrane fusion for entry into host cells. These findings indicate that bats constitute a potentially important and likely ancient reservoir for a diverse pool of influenza viruses.