Crop residues can be used for biogas production in farm scale reactors. Use of a process temperature below mesophilic conditions reduces the need for heating as well as investment and operating costs, although it may also reduce the methane yield. In the present study the effect of temperature on net energy output was studied using sugar beet tops and straw as substrates for two pilot-scale reactors. Digestion was found to be stable down to 11 degrees C and optimal methane yield was obtained at 30 degrees C. The methane yield and process performance was studied at 15 degrees C and 30 'C as organic loading rates were increased. It was found that the highest net energy production would be achieved at 30 degrees C with a loading rate of 3.3 kg VS m(-3) day(-1). Running a low-cost process at ambient temperatures would give a net energy output of 60% of that obtained at 30 degrees C. (c) 2006 Elsevier Ltd. All rights reserved.