In order to enhance the efficiency of anaerobic digestion, the effects of ultrasonic pretreatment have been studied on waste-activated sludge. Solubilisation of chemical oxygen demand (COD), solid and nitrogen has been proved. Flocs were broken and compounds were made soluble. In the same time, particle size decreased with specific energy applied. In terms of biodegradability, ultrasound led to an increase in biogas production. Moreover, the relationship between biogas production and sludge fractions has been examined. For specific energy input lower than 3000 kJ/kg of total solids, biogas production linked to the particulate fraction of sludge was constant, even if the solids concentration decreased. On the other hand. biogas production linked to the soluble part of sludge increased with ultrasonic power. (C) 2004 Elsevier B.V. All rights reserved.