Conventional activated sludge wastewater treatment plants currently produce a large quantity of excess sludge. To reduce this sludge production and to improve sludge characteristics in view of their subsequent elimination, an ultrasonic cell disintegration process was studied. In a lab‐scale continuous flow pilot plant, part of the return sludge was sonicated by low‐frequency and high‐powered ultrasound and then recycled to the aeration tank. Two parallel lines were used: one as a control and the other as an assay with ultrasonic treatment. The reactors were continuously fed with synthetic domestic wastewater with a COD (chemical oxygen demand) of approximately 0.5 g l−1 corresponding to a daily load of 0.35–0.50 kg COD kg−1 TS d−1. Removal efficiencies (carbon, particles), excess sludge production and sludge characteristics (particle size distribution, mineralization, respiration rate, biological component) were measured every day during the 56‐day experiment. This study showed that whilst organic removal efficiency did not deteriorate, excess sludge production was decreased by about 25–30% by an ultrasonic treatment. Several hypotheses are advanced: (i) the treatment made a part of the organic matter soluble as a consequence of the floc disintegration, and optimised the conversion of the carbonaceous pollutants into carbon dioxide and (ii) the treatment modified the physical characteristics of sludge by a mechanical effect: floc size was reduced, increasing the exchange surface and sludge activity. The originality of this study is that experiments were conducted in a continuous‐flow activated sludge reactor rather than in a batch reactor.