Three model peptides have been studied in an effort to understand the molecular basis for the fusogenic potency of foamy virus, These peptides corresponded to a 23 amino acid helical segment close to the amino terminus, a shortened 17 amino acid, more hydrophobic homolog of this peptide, and an 18-amino-acid peptide close to or within the transmembrane domain. The peptides have a conformation containing both alpha -helical and beta -structure in aqueous solution but are predominantly alpha -helical in solutions of trifluoroethanol, as assessed by circular dichroism, In common with other viruses, the most fusogenic peptide was the one closest to the amino terminus, However, unlike several other fusion peptides that have been studied previously, this peptide did not promote increase negative membrane curvature as assessed by effects of the peptide on lipid polymorphism. Nevertheless, the foamy virus fusion peptide promotes membrane fusion, apparently by a mechanism that is independent of changes in membrane curvature, We demonstrate that there is a synergistic action in the promotion of membrane fusion between the peptide from the amino terminal region and the one from the region adjacent to the transmembrane segment.