The present invention provides an isolated mammalian negative strand RNA virus, metapneumovirus (MPV), within the sub-family Pneumoviridae , of the family Paramyxoviridae . The invention also provides isolated mammalian negative strand RNA viruses identifiable as phylogenetically corresponding or relating to the genus Metapneumovirus and components thereof. In particular the invention provides a mammalian MPV, subgroups and variants thereof. The invention relates to genomic nucleotide sequences of different isolates of mammalian metapneumoviruses, in particular human metapneumoviruses. The invention relates to the use of the sequence information of different isolates of mammalian metapneumoviruses for diagnostic and therapeutic methods. The present invention relates to nucleotide sequences encoding the genome of a metapneumovirus or a portion thereof, including both mammalian and avian metapneumovirus. The invention further encompasses chimeric or recombinant viruses encoded by said nucleotide sequences. The invention also relates to chimeric and recombinant mammalian MPV that comprise one or more non-native or heterologous sequences. The invention further relates to vaccine formulations comprising mammalian or avian metapneumovirus, including recombinant and chimeric forms of said viruses. The vaccine preparations of the invention encompass multivalent vaccines, including bivalent and trivalent vaccine preparations.