The ganglioside GD1a, which serves as a receptor for Sendai virus, also affects lipid polymorphism as determined by 31P nuclear magnetic resonance. The ganglioside promotes the formation of isotropic structures in monomethyldioleoylphosphatidylethanolamine. GD1a also raises the bilayer to hexagonal phase transition temperature of this lipid. The effects of GD1a on the kinetics of viral fusion can be understood on the basis of its role in facilitating the binding of Sendai virus to target membranes as well as its effects on membrane physical properties. Fusion of Sendai virus with liposomes composed of egg phosphatidylethanolamine is particularly sensitive to the presence of ganglioside. In the absence of ganglioside no fusion is observed due to the absence of virus binding to the target membrane. Between 2 and 6 mol % GD1a in egg phosphatidylethanolamine liposomes there is a marked increase in the rate constant of binding of the virus to the liposome but a decrease in the fusion rate constant. The latter effect is found to be common to a number of other amphiphiles that raise the bilayer to hexagonal phase transition temperature. The ganglioside enhances virus binding to liposomes of all the compositions studied, but leakage rates and fusion rate constants are either unaffected or reduced. In the systems studied, the enhanced formation of isotropic structures in liposomes containing the ganglioside does not enhance the kinetics of the actual fusion reaction.