The effects of a Naja mossambica mossambica cardiotoxin on the thermotropic properties of charged phospholipids have been studied by fluorescence polarization, differential scanning calorimetry, and Raman spectroscopy. The binding of the toxin is only governed by the net charge at the interface and is not affected by the polar head group structure of the phospholipids or by the acyl chains physical state, degree of insaturation, or length. The effect of the toxin on the phospholipid structure is drastic. In all cases, the gel to liquid-crystalline phase transition monitored by fluorescence and Raman spectroscopies is progressively abolished without notable shift in temperature as the proportion of toxin is increased. The endothermic peaks detected by differential scanning calorimetry decrease in intensity as the toxin content is increased but always remain sharp. All the techniques used give complementary results, and none of them reveals the presence of secondary transitions at higher or lower temperatures. We thus believe that the lipid molecules that are perturbed by the toxin, approximately 10 +/- 2 molecules, do not undergo a phase transition. Raman results demonstrate that these "boundary" lipids display a population of gauche rotamers that is as high as the one found in the liquid-crystalline phase of the pure phospholipid and this even well below the phase transition temperature. On the other hand, fluorescence results are interpreted as due to a partial immobilization of the lipids in contact with the toxin above the transition temperature. Thus, even though the interaction is governed by electrostatic forces, the toxin penetrates at least partially into the bilayers, inducing a disorganization of the aliphatic chains and changes in their mobility; this could explain their lytic activity.