Pilot plant studies on biological filter treatment for musty odor in drinking water were conducted. The plant was operated for 2 years at a filtration rate of 170 m/d and the surface water of Lake Biwa was employed as an influent. 2-Methylisoborneol (MIB) produced by Oscillatoria tenuis was the main cause of the musty odor. In the season of natural musty odor generation, total MIB concentration and the insoluble to total MIB concentration ratio in the influent varied to a large extent, but removal efficiencies of MIB by the filter were about 60 to 80%. Out of the season of natural musty odor, reagent MIB was added to the influent and the removal efficiencies obtained were about 70 to 80%. In addition to MIB, color, turbidity, Fe, Mn, NH4-N and KMnO4 consumption value were also effectively removed by the biological filter. Using the effluent from the filter, jar tests for powdered activated carbon treatment and coagulation-sedimentation treatment were conducted. From the results, the biological filter treatment seemed to effectively decrease the powdered activated carbon consumption even in cases where powdered activated carbon additions might be needed. No ill effect on coagulation-sedimentation treatment was seen and the required coagulant dosage was less in water after biological filter treatment.