A fundamental study was carried out to assess the relationship between the availability of dissolved oxygen in a laboratory-scale biofilm reactor with some important system performance parameters such as biofilm growth rate, organic stabilization rate, sulfide production potential, aerobic layer-thickness, film density, and biofilm sloughing patterns. The feed solution was filtered sewage supplemented with glucose to maintain an initial COD of 300 mg/l. Four DO levels were examined: 1, 3, 5 and 8 mg/l. The test apparatus consisted of annular reactors, each having outer stationary and an inner rotating cylinders, so that the hydraulic shear throughout the reactor was uniform. The liquid space between the two concentric cylinders was 1.0 cm and the total growth surface was 1,200 cm2, while the liquid volume inside the reactor was 755 cm3. The rotating speed was controlled at 14 cm/s (0.46 ft/s) and the operating temperature was controlled at 20°C. The results of this study have indicated that an increase of DO tends to increase the cell yield, the maximum active (or aerobic) layer thickness, as well as the COD stabilization rate. However, DO shows no apparent effect on the film's volumetric dry density. Instead. it is a function of the film thickness; i.e., the thicker film has a higher density. For a mature biofilm, the ratio of the aerobic to the total film thickness increases linearly from 0.08 to 0.27 when the DO is increased from 1 to 8 mg/l. The maximum COD removal rate at 8 mg/l DO and an initial COD of 300 mg/l is about 1.5 g/h/m2. Sulfide production starts to occur when the film thickness reaches 200 μm and the DO is at 1.0 mg/l. As the DO is increased to 8.0 mg/l, sulfide production stili occurs when the film thickness reaches 700 μm. Therefore, odour production seems to be an inherent problem associated with the biofilm treatment system. The pattern of the biofilm sloughings seem to follow a 40- to 45-day cycle regardless of the operating DO. In most cases, massive and patchy sloughings occur for two to three days; beyond that, a gradual but steady redevelopment of the biofilm takes place. During times of sloughing, the overall treatment efficiency can be slightly affected, primarily due to the presence of high concentrations of SS in the treated effluent.