This work examines the potential for methane production from anaerobic co-digestion of olive mill wastewater and wine-grape residues with slaughterhouse wastewater. Continuous (mesophilic) and batch (mesophilic and thermophilic) experiments were studied, both with the separate types of by-products and with mixtures. Methane yields from olive oil wastewater, winery residues and slaughterhouse wastewater were 108, 147 and 297 L CH4 kg(-1) COD fed respectively. Co-digestion with 50% olive oil wastewater and 50% slaughterhouse wastewater or 50% winery residues gave a methane yield of 184 and 214 L CH4 kg(-1) COD, respectively. Furthermore, the methane yield was 188 L CH4 kg(-1) COD added, co-digesting a mixture of 50% winery residues and slaughterhouse wastewater. Finally, the same mixtures under thermophilic conditions gave methane yields of 282, 301 and 219 L CH4 kg(-1) COD, respectively. These results suggest that methane can be produced very efficiently by co-digesting olive oil wastewater, wine-grape residues and slaughterhouse wastewater. (C) 2007 Elsevier Ltd. All rights reserved.