Sewage sludge is a hazardous waste, which must be managed adequately. Mesophilic anaerobic digestion is a widely employed treatment for sewage sludge involving several disadvantages such as low methane yield, poor biodegradability, and nutrient imbalance. Tomato waste was proposed as an easily biodegradable co-substrate to increase the viability of the process in a centralized system. The mixture proportion of sewage sludge and tomato waste evaluated was 95:5 (wet weight), respectively. The stability was maintained within correct parameters in an organic loading rate from 0.4 to 2.2 kg total volatile solids (VS)/m(3) day. Moreover, the methane yield coefficient was 159 l/kg VS (0 A degrees C, 1 atm), and the studied mixture showed a high anaerobic biodegradability of 95 % (in VS). Although the ammonia concentration increased until 1,864 A +/- 23 mg/l, no inhibition phenomenon was determined in the stability variables, methane yield, or kinetics parameters studied.