Two lab-scale mesophilic (MAF) and thermophilic (TAF) anaerobic filters treating effluents from a mussel cooking factory were operated at their maximum organic load rate (OLR) for a long period of time. The biomass profiles and the distribution of occluded and attached biomass were determined. Biomass distribution in the MAF was more homogeneous along the filter. Most of the biomass in the TAF was attached while both attached and occluded biomass was observed in the MAF. The hydrolytic, acidogenic, acetogenic and methanogenic activities of sludges at different levels were determined. An attempt is made to explain the behaviour of both reactors as a function of these activities and other kinetic parameters.