The mesophilic anaerobic treatment of concentrated sludge from an Atlantic salmon smolt hatchery (total solids (TS): 6.3-12.3wt%) was investigated in a continuous stirred tank reactor (CSTR) at 35 degrees C and 55-60 days hydraulic retention time (HRT). COD-stabilization between 44% and 54% and methane yields between 0.140 and 0.154l/g COD added (0.260-0.281l/g VS added) were achieved. The process was strongly inhibited, with volatile fatty acid concentrations of up to 28 g/l. But the buffer capacity was sufficient to keep the pH-value at 7.4-7.55 during the whole operation. The fertilizing value of the treated sludge was estimated to be 3.4-6.8 kg N and 1.2-2.4 kg P per ton. However, the high VFA content would necessitate special means of application. The energy from the methane that was achieved in the present study would be sufficient to cover about 2-4% of the energy demands of a flow-through hatchery.