Two lab-scare single-fed (SFR) and multi-fed (MFR) upflow anaerobic filters treating a cheese whey wastewater were operated at Organic Loading Rates (OLR) higher than 20 kg COD/m(3)/d. The occluded and attached biomass values and the hydrolytic, acidogenic and methanogenic activities of sludges at different heights were determined in the filters. The feeding policy affected both biomass concentration (attached and occluded) and activity. Although a lower amount of biomass was retained in the MFR, the distribution along the filter was more homogeneous than in the SFR. The specific activities of the different trophic groups were higher in the MFR. Because of both facts, the methanogenic capacity of the MFR was higher than that in the SFR, this resulting in a more efficient operation, especially when operated at the highest OLR.