The anaerobic treatment of liquid fraction of hen manure was studied in two parallel laboratory 2.61 UASB reactors at 35 degrees C. The granular sludge from a laboratory UASB reactor treating synthetic wastewater was used as a seed sludge. The liquid fraction of hen manure was obtained directly from an industrial farm located in Moscow region after separation of diluted manure in situ into solid and liquid fractions. Before feeding, the waste water was preacidified under microaerobic conditions at laboratory ambient temperature (18-20 degrees C) for 1-2 days. The influent GOD was 10.5-20 g/l of which volatile fatty acids (VFA) were 3.9-6.7 g COD/l, pH was between 6.2-7.1. The initial organic loading rate (OLR) was Ig COD/l day(-1) then it was stepwise increased to the values of II-I COD/l day(-1). Under these OLR and hydraulic retention rime of 1-2 days, both reactors demonstrated optimal stability with treatment efficiencies out total COD reduction of 70-75%. The biogas production rate at the OLR of 11-12 g COD/l day(-1);vas 3.5-3.6l/l day(-1) (methane content - 79-81%) and effluent pH of about 8.