The mixture of olive mill wastewater (OMW) with abattoir wastewater (AW) induced the precipitation of OMW phenolic compounds with AW proteins and the removal of chemical oxygen demand (COD). Anaerobic batch experiments with different proportions of OMW:AW (v:v) (10:90; 20:80; 40:60; 60:40) showed that the 40:60 mixture can be co-digested in a continuous system because it improves the C/N ratio and reduces the inhibition of methanogenic bacteria by phenolic compounds. The continuous co-digestion of this mixture was tested under mesophilic (37 degrees C and thermophilic (55 degrees C conditions using an upflow anaerobic filter (UAF). The organic loading rates (OLRs) which could be achieved during the co-digestion were higher than those obtained during the anaerobic treatment of OMW alone. The change from a mesophilic to a thermophilic environment in the UAF was carried out with a short start-up of thermophilic condition. A higher discoloration, higher COD removal (80%), and biogas yield (0.52 L/g of COD removal) were obtained at 55 degrees C and the performance of the reactor was maintained even at an OLR as high as 12 g of COD/L.d. The reduction of faecal coliforms achieved after digesting the influent was important in mesophilic (11-21 MPN/mL) and thermophilic conditions (0-11 MPN/mL) and influenced by the combined effect of the antimicrobial activity of OMW and thermophilic operating conditions. The enhanced performance obtained with the UAF treating OMW:AW (40:60) could be attributed to the dilution of OMW with AW, which reduces the toxicity, and the thermophilic conditions, which improves the biodegradability.