The nucleotide sequence of the hemagglutinin-neuraminidase (HN) gene of human parainfluenza type 2 virus (PIV-2) was determined. The PIV-2 HN gene was 2112 nucleotides excluding poly(A) tail. There was a single large open reading frame in the mRNA which encoded a protein of 571 amino acids with a calculated molecular weight of 63,262. Analysis of the deduced amino acid sequence revealed that there were fourteen potential glycosylation sites and a major hydrophobic region near the N-terminus, which would anchor the protein in the viral membrane. Comparisons of the HN protein sequences of PIV-2 with those of Simian virus 5 (SV5), Sendai virus (SV, parainfluenza virus type 1), human parainfluenza virus type 3 (PIV-3), type 4 (PIV-4), bovine parainfluenza virus type 3 (BPIV-3), mumps virus (MuV), and Newcastle disease virus (NDV) showed definite amino acid sequence relatedness, indicating a common ancestor for these viruses. Furthermore, statistical analysis of the protein sequences suggested a possible evolutionary relatedness among the paramyxoviruses. This is the first time that a phylogenetic tree has been constructed for all the parainfluenza viruses and mumps virus which are infectious to humans. In addition, amino acid sequences involved in hemagglutinating and neuraminidase activities of paramyxovirus were discussed.