The 'fusion peptides' of a group of enveloped viruses that includes influenza, paramyxo-, retro-and filo-viruses are N-terminal regions of their membrane fusion proteins generated by cleavage of non-functional precursors. For the influenza membrane fusion protein, haemagglutinin (HA), the three-dimensional structures of precursor HA, cleaved HA and fusion-activated HA show that the fusion peptides are located in different positions in all three forms and adopt different structures. Analyses of mutant HAs with changes in fusion peptide sequence indicate the importance of specific residues for membrane-fusion activity and suggest a structure for the fusion peptide in a fusion-active molecule.