The effects of the insecticides malathion and deltamethrin on the phase behaviour of multilamellar dimyristoylphosphatidylcholine (DMPC) bilayers were studied by differential scanning calorimetry. The results show that both insecticides broaden the main transition and suppress the main transition temperature. High concentrations of malathion narrow the main transition compared to lower concentrations of malathion. The C_p curves for lipid bilayers with high concentrations of malathion show several peaks in the main transition region, suggesting that more than one phase is present. For malathion interacting with DMPC multilamellar vesicles a high-temperature feature developed with increasing insecticide concentration. This was not seen for the vesicles with deltamethrin. Malathion affects the pretransition by broadening it and by lowering the transition temperature and the transition enthalpy. Small concentrations of deltamethrin make the pretransition disappear, but a minor peak in the specific heat appears at the same position for higher concentrations of deltamethrin. Although both insecticides are hydrophobic molecules their effects on DMPC multilamellar bilayers are very different. This suggests that details of the molecular structure of hydrophobic molecules are important for their nonspecific interactions with lipid bilayers.