The interaction of surfactants with the vesicle membrane of the negatively charged lipid, dilauroylphosphatidic acid, was investigated through their effect on the gel-to-liquid-crystaifine phase transition of the lipid bilayer. Three types of surfactants (anionic, cationic and non-ionic) with different hydrocarbon chain length were examined. (i) Anionic sodium aikylsulfates affected the phase transition temperature, T, only weakly. (ii) Non-ionic alkanoyl-N-methylglucamides decreased T monotonously with increasing concentration. The depression of T induced by these surfactants was analyzed by applying the van't Hoff model for the freezing-point depression, and the partition coefficients of the surfactants between bulk water and lipid membrane were estimated. (iii) Cationic alkyltrimethylammonium bromides affected T in a complex manner depending on the hydrocarbon chain length of the surfactants. Octyl-/tetradecyl-trimethyiammonium bromide depressed/elevated T monotonously with increasing concentration, whereas the change in T induced by decyl-and dodecyltrimethylammonium bromides was not monotonous but biphasic. This complex behavior of the phase transition temperature was well explained, based on the statistical mechanical theory presented by Suezaki et al. (Biochim. Biophys. Acta, 818 (1985) 31--37), which takes into account the interaction between surfactant molecules incorporated in the lipid membrane.