It is generally known that unilamellar vesicles prepared from zwitterionic phosphatidylcholines (PCs) do not aggregate in the presence of multivalent cations, as opposed to the case of electrostatically stabilized vesicles of acidic phospholipids. However, we have found that Be2+ can induce the aggregation of PC vesicles. The aggregation exhibited characteristic features concerning the Be2+ concentration and temperature, as follows : (i) there exists an optimum concentration range of Be2+ to induce the aggregation, and (ii) the aggregation becomes most pronounced at the temperature corresponding to the bilayer phase transition of the vesicle membranes; it is suppressed completely at higher temperatures. In addition, the Be2+-induced aggregation of PC vesicles was found to be reversible with respect to the Be2+ concentration and temperature. The reversibility of the aggregation suggests that vesicles are trapped in a secondary minimum of intervesicular potential in the aggregating entities. This in turn leads to the speculation that the repulsive hydration force characteristic of PC vesicles is partially destroyed by the action of Be2+.