We have studied the effects of gangliosides on the polymorphic behaviour of phosphatidylethanolamines. The ganglioside G D 1 a promotes the formation of phases which give rise to isotropic 3 1 P-NMR resonance lines, particularly in the temperature range of the L α to H I I transition. In addition, higher mol fractions of ganglioside raise the L α to H I I phase transition temperature. Our results demonstrate that small mol fractions of gangliosides can have profound effects on the molecular organization of phosphatidylethanolamines.