The nucleotide sequence of the mRNA coding for the fusion glycoprotein (F) of the paramyxovirus, simian virus 5, has been obtained. There is a single large open reading frame on the mRNA that encodes a protein of 529 amino acids with a molecular weight of 56,531. The proteolytic cleavage/activation site of F, to yield F2 and F1, contains five arginine residues. Six potential glycosylation sites were identified in the protein, two on F2 and four on F1. The deduced amino acid sequence indicates that F is extensively hydrophobic over the length of the polypeptide chain. Three regions are very hydrophobic and could interact directly with membranes: these are the NH2-terminal putative signal peptide, the COOH-terminal putative membrane anchorage domain, and the NH2-terminal region of F1.