Low-frequency and high-intensity ultrasonic treatment of sewage plant sludge disrupts the flocs and lyses the bacterial cells. This results in a substantial reduction in the volume of the floes and a release of both inter and intracellular materials. The evolution of the particles size (floes and isolated microorganisms) is evaluated by a Malvern Mastersizer gramilometer and the release of material is quantified by measuring the chemical oxygen demand and the proteins solubilized in the solution. In the first part, the disruption of the particles was undertaken with low concentrated sludge from an aeration tank in order to comprehend better the mechanisms and to modelize them. In the second part, the influence of the initial concentration is studied by working with higher concentrated sludge from a settler. Ultrasonic treatment also causes a change in the settleability and filterability of the sludge which the effects are evaluated in the last part. (C) 2003 Elsevier Science B.V. All rights reserved.