A library of cDNA clones from mRNA of human parainfluenza type 2 virus (PIV2) was constructed and the nucleotide sequence of the fusion (F) glycoprotein gene determined. The F gene boundaries were obtained by primer extension sequencing on F mRNA and on viral genomic RNA. The mRNA coding for the F glycoprotein is composed of 1918 nucleotides. It contains a single large open reading frame that encodes a protein of 551 amino acids with an Mr of 59586. The predicted PIV2 F protein contains the cleavage-activation site (amino acids 101 to 106) including two Arg and two Lys residues, where the protein is cleaved by host protease into F1 and F2 subunits by analogy with other paramyxoviruses. Three hydrophobic domains are recognized, the signal peptide (amino acids 1 to 21), that is cleaved off in the mature protein, the fusion peptide (amino acids 107 to 132) at the cleavage-generated N terminus of subunit F1 and the membrane anchorage region (amino acids 486 to 513) near the C terminus of the protein. The predicted F protein has six potential glycosylation sites and 10 of the 12 Cys residues present have conserved positions as compared with those of other paramyxovirus F proteins. Primer extension sequencing on viral RNA gave the 3' end sequence as UAAAUUCU6 followed by the 5' end of the haemagglutinin-neuraminidase (HN) gene thus establishing the order of genes coding for the viral glycoproteins of PIV2 as 5'-F-HN-3'.