Laboratory experiments were carried out to simulate the degradation of organic materials in rivers. Glucose was fed to a complete mixed reactor continuously where an attached biofilm was developed. Soluble COD of the effluent was almost constant (10-15 mg/l) under various COD loadings up to 12.3 g/m²·d, and did not decrease when the hydraulic retention time was extended. Gel chromatography of the substrate and the effluent revealed that organic compounds whose molecular weight were as low as that of glucose remained in the effluent.·The attached biomass was 2-15 g/m² irrespective of the glucose loading. These values were equivalent to 44-326 mg/l-reactor volume, which were much larger than the suspended biomass. The number of heterotrophic bacteria in the attached biofilm increased with the increase of glucose loading, and was about 100-1000 times that in the effluent. Oxygen consumption by the attached biofilm was estimated from oxygen balance around the reactor, and it was 0.65 g/g-soluble COD removed, which suggests that the growth yield of the biofilm was 0.35. The endogenous respiration required 98 mg-oxygen/ g-attached biofilm · day.