This paper presents the results obtained for the digestion of primary sludge (PS) and co-digestion of this sludge with the fruit and vegetable fraction of municipal solid wastes (FVFMSW) under mesophilic conditions. This mixture was prepared with a PS content of 22%. The anaerobic digestion process was evaluated under static conditions and with different mixing conditions, with good results being found for the digesters with limited mixing, this representing an energy saving. The results for co-digestion of mixtures of PS +FVFMSW are better than those obtained from digestion of PS on its own. Biogas production for co-digestion is much greater thanks to the larger volatile-solid (VS) content of this feedstock. Nevertheless, biogas yield and specific gas production for the two digestion processes are similar, with values in the range 0.6-0.81 g(-1) VS destroyed for the first parameter and in the range 0.4-0.61 g(-1) VS fed for the second. The co-digestion process was also evaluated at different organic loading rates (OLR) under low mixing conditions, with stable performance being obtained even when the systems were overloaded. (c) 2005 Elsevier Ltd. All rights reserved.