Some existing anaerobic digesters treating sewage sludge have a non-used capacity. The use of this extra capacity by introducing additional wastes to conduct the co-digestion could enhance biogas production and plant economic feasibility. Fruit wastes from the food industry could be proper co-substrates due to their high biodegradability, but the harvesting seasons require the use of different kind of fruits causing many transitory conditions throughout the year. Two lab-scale continuous anaerobic digesters treating sewage sludge were operated, one as a reference reactor and the other one as a co-digester. The transitory state was evaluated when fruit waste supply was started, when the co-substrate was changed (peach, banana and apple waste) and when fruit waste supply was stopped. In the transition from mono- to co-digestion, volatile fatty acids concentration rose from 0.07 to 1.70 g L-1 due to the OLR increase, but this situation was recovered in only 5 days. The introduction of different kind of fruit wastes resulted in an alteration of alkalinity, without affecting volatile fatty acids concentration, and in an increase of methane production between 110% and 180% depending on the characteristics of the co-substrate. Finally, when co-digestion was stopped, the parameters converged, at different rates, to the values recorded in the reference digester. It could be concluded that the change of one co-substrate by another one of the same type did not lead to system instability. (C) 2014 Elsevier B.V. All rights reserved.