The aim of this study was to investigate the effect of temperature and hydraulic retention time (HRT) on anaerobic co-digestion/biomethanation of cattle manure and agricultural residues (clover, grass and wheat straw). For this purpose, 12 semi-continuous reactors, fed with/without agricultural residues, were operated under varied temperature (10 +/-, 20 +/- and 35 +/- 1 degrees C) and HRT (20 and 30 days) conditions. During the experimental study, all reactors were fed once on a daily basis and operated with an organic loading rate of 3 g volatile solids (VS)/L x d. Daily biogas production, pH, biogas composition, volatile fatty acids, chemical oxygen demand and solids' (dry matter andVS) concentrations were analyzed. Results indicate that the effect of agricultural residue addition did not influence the rate and extent of biomethanation of cattle manure. An effect of temperature was clearly observed on reactor performance for both operational HRTs of 20 and 30 days. At 35 +/- 1 degrees C, reactors produced 299-324 mL biogas/g VS added, whereas this value remained between 87-138 mL biogas/g VS for the reactors run at 20+ degrees C. The results were comparable to the studies performed on anaerobic digestion of cattle manure in terms of both methane production yield (39-182 mL CH4/g VS added) and dry matter reduction efficiencies (33-51%).