In this study, two identical lab-scale Upflow Anaerobic Sludge Bed Reactors (UASBR) with an effective volume of 6.45 1 were used and operated as continuous systems for about 12 months. The mesophilic UASBR was operated at 35degrees+/-2.0 degreesC, whereas the psychrophilic UASBR was operated at room temperature to start with and reduced to around 10 degreesC step by step. In the study, synthetic wastewater characterizing domestic sewage was fed to the reactors. Results indicated around 80% COD removal at 16 degreesC at a HRT of 4.8 It, while at the same HRT the COD removal could be observed above 85% at 35 degreesC. The total biogas productions were observed about the same amount between the HRT's of 10-4 h in both reactors. However, when the temperature in the psychrophilic reactor was reduced to 10 degreesC, total biogas production decreased significantly at the beginning, but improved afterwards. The average methane content in the psychrophilic reactor was always found higher (93%) than it was in the mesophilic reactor (90%). Although the COD removal at 10 degreesC decreased to 74%, anaerobic treatment of low strength wastewaters could be well established and a more feasible system due to no heating requirement could be achieved successfully.