Material balance has been used to evaluate the COD behaviour and the time required for fed-batch digestion of mixtures of domestic sludges and pre-hydrolysed agricultural wastes. Pre-hydrolysis of the feed materials has been used to penetrate the strong lignocellulosic structure of these wastes as well as to increase the fraction of soluble organic substances in the mixture. The influence of the organic loading rate on the main process parameters (methane, carbon dioxide, total biogas productions and their respective conversion yields) has also been investigated. The organic load has been varied from 0.8 up to 6.1 g(COD) dm(-3) day(-1), corresponding to a range of volatile solids load of 0.6-4.5 g(VS) dm(-3) day(-1) for the material under consideration. These values are slightly higher than those usually employed in conventional digester for domestic sewage sludges. However, methane production reached a maximum rate of only 5.6 mmol dm(-3) day(-1) at an organic loading rate of 4.6 g(COD) dm(-3) day(-1), while both CH4 content and production of biogas rapidly fell over 2.2 g(COD) dm(-3) day(-1). On the whole, these results suggest that removal of lignin is necessary in order to carry out the continuous anaerobic digestion of pre-hydrolysed agricultural wastes rich in woody materials.