Large unilamellar vesicles composed of a nonionic synthetic glycolipid, 1,3-di-O-phytanyl-2-O-(¯-D-maltotriosyl)glycerol exhibited a pH-dependent aggregation-disaggregation process; vesicle aggregation occurred in a lower pH region and vesicle disaggregation occurred in a higher pH region. This process was almost reversible and the aggregation threshold pH increased as NaCl concentration increased. Electrophoretic mobility measurements revealed that the glycolipid vesicles are negatively charged in the range pH 1.6-13. The change in ³-potentials as functions of pH and NaCl concentration could be well described by the Gouy-Chapman expression of the surface charges with an assumption that the interfacial charges arise from the "adsorption" of OH¡ at the vesicle-water interface and the dissociation of hydroxyl groups of the sugar headgroup in a higher pH regime (>pH 10). The pHdependent aggregation process was reasonably well described by the classical DLVO theory. Thus, the double-layer repulsive forces appear to be a major factor in stabilizing the glycolipid vesicle suspension.