The action of allethrin, a pyrethroid with an alkenylmethylcyclopentenolone group, on the thermotropic properties of lipid vesicles has been investigated. Application of turbidimetry and differential scanning calorimetry to liposomes made with dimyristoyl- (DMPC), dipalmitoyl- (DPPC), and distearoyl- (DSPC) phosphatidycholine, containing variable concentrations of allethrin, showed that the pyrethroid lowers the temperature at which the phase transition of the phospholipid occurs. Furthermore, allethrin produces a broadening of the peak which marks the gel to liquid-crystalline phase transition. The appearance of a second peak as the allethrin concentration in the membranes rises indicates a limited miscibility of the pyrethroid in lipids. Incorporation of allethrin in carboxyfluorescein-trapped unilamellar liposomes, followed by incubation at several temperatures, enhances carboxyfluorescein release in allethrin-containing vesicles. The results are discussed in terms of a preferential localization of allethrin in an extended orientation in the bilayer with the carbonyl group of the alkenylmethylcyclopentenolone residue in the lipid water interface and the cyclopropanecarboxylate moiety between the hydrocarbon acyl chain of the phospholipids.