The dielectric and conductometric properties of mixed ganglioside/phospholipid vesicle aqueous suspensions were measured in the frequency range from 10 kHz to 10 MHz, where a pronounced dielectric dispersion due to surface bilayer polarization occurs. The concentration of the ganglioside employed, a disialoglycosphingolipid GD1a, was varied from 50 to 500 mu g/mL (corresponding to a molar ratio [GD1a]/[DPPC] from 0.2 x 10(-3) to 2 x 10(-3)) and the excess surface polarizability, attributed to ganglioside organization within the lipid bilayer, was evaluated. The dependence of the ganglioside concentration shows that a clusterization process occurs, giving rise to the formation of large in-phase aggregates whose coordination number, of the order of a few hundred dipoles, depends on temperature. The relevance, from a biological point of view, of a nonuniform distribution of GD1a in a lipid bilayer membrane is briefly discussed.