Ultrasound at 20 Hz was applied at different energy levels (Es) to treat Scenedesmus biomass, and organic matter solubilization, particle size distribution, cell disruption and biochemical methane potential were evaluated. An Es of 35.5 and 47.2 MJ/kg resulted in floc deagglomeration but no improvement in methane production compared to untreated biomass. At an Es of 128.9, cell wall disruption was observed together with a 3.1-fold organic matter solubilization and an approximately 2-fold methane production in comparison with untreated biomass. Thermal pretreatment at 80 degrees C caused cell wall disruption and improved anaerobic biodegradability 1.6-fold compared to untreated biomass. Since sonication caused a temperature increase in samples to as high as 85 degrees C, it is likely that thermal effects accounted for much of the observed changes in the biomass. Given that ultrasound treatment at the highest Es studied only increased methane production by 1.2-fold over thermal treatment at 80 degrees C, the higher energy requirement of sonication might not justify the use of this approach over thermal treatment.