The objectives of this study are to evaluate the distribution of specific activity within attached biofilm in afluidized bed and to investigate the detachment of each “active” biomass from the biofilm. A fluidized bed was operated in asteady state as a methanogenic reactor with supply of a synthetic wastewater, a mixture of acetate, propionate, and butyrate.Batch experiments were conducted to measure the specific activities of biomass in effluent, stepwise sloughed biomass, andattached biomass in bed. The results indicated that the biomass in the surface layer of biofilm had much higher activity thanthat inside the layer. The highest activity was obtained for biomass in effluent. The distributions of the activities illustrated thatmore “active” biomass existed in the biofilm surface where more frequent detachment of biomass occurred.The trend of the activity distribution was explained by a newly developed model considering accumulation of inert biomassand biomass detachment from biofilm surface. A first order rate constant of detachment for each “active” biomass was roughlyestimated at 0.013, 0.022, and 0.055 [day^-1] for acetate, propionate, and butyrate decomposing biomasses, respectively. Theestimated values were much larger than literature values based on whole biomass. The model predicts that bacteria with highergrowth rates are able to exist predominantly in the surface layer against high biomass detachment and that the decay biomassmainly accumulates inside the biofilm.