Provided is an isolated mammalian negative strand RNA virus, metapneumovirus (MPV). Described are isolated mammalian negative strand RNA viruses identifiable as phylogenetically corresponding or relating to the genus Metapneumovirus and components thereof. Also described is mammalian MPV, subgroups and variants thereof, and genomic nucleotide sequences of different isolates of mammalian metapneumoviruses, in particular human metapneumoviruses. The use of the sequence information of different isolates of mammalian metapneumoviruses for diagnostic and therapeutic methods is provided. Nucleotide sequences encoding the genome of a metapneumovirus or a portion thereof, including both mammalian and avian metapneumovirus are described. Also provided are chimeric or recombinant viruses encoded by said nucleotide sequences and chimeric and recombinant mammalian MPV that comprise one or more non-native or heterologous sequences.