A method and apparatus for and aggregates in a fluid medium. The present invention employs an ultrasonic device to efficiently breakup particle agglomerates by driving the ultrasonic signal over a small range of frequencies around the acoustic slow wave frequency of the saturated agglomerate. At this frequency, the fluid vibrates out of phase with the solid and is forced out through the pore structure in the agglomerate. This relative fluid motion to exert high viscous stresses at the particle-particle contact points which leads to fracture of the agglomerate and the redispersion of the individual particles. The apparatus includes a dispersing vessel containing aggregates of particles in a fluid, a sonic member for applying an ultrasonic signal in said dispersing vessel for separating the aggregates to form dispersed particles.