The aggregation of fine ash particles has an important role in controlling the deposition of widely dispersed volcanic ash. Here we report measurements of electrical charge on ash particles falling from the eruption columns of Sakurajima volcano in Japan. Absolute charge to mass (q/m) ratios ranged from +3 to +6x10^-4 C kg^-1 and from -2 to -5x10^-4 C kg^-1. The average q/m ratio ranged from +2 to +5x10^-5 C kg^-1. The generation of electrostatic charge may result from triboelectric effects in the plume, or from fracture-induced charging. Charge on ash particles provides attractive forces large enough to cause the aggregation of smaller particles and the adhesion of dust to larger particles. Particle aggregation may explain the polymodal grain-size distributions commonly found in ash-fall deposits, and the proximal deposition of fine ash, as well as the distal deposition of coarse particles in these deposits. Our data suggest that electrostatic effects greatly influence the dispersal and deposition of ash during explosive volcanic eruptions.