This paper addresses the anaerobic digestion of the solid fraction of pig farm slurries that was carried out at a laboratory scale. The efficiency of a biological treatment process corresponds with the transformation of the organic substances and the size of the particles. This study discusses the transition of particle size fractions of the piggery slurry in anaerobic degradation. The experiment only included particles retained on a I mm-mesh hydraulic filter. The original substratum was divided in 800 ml-volume aliquots and each was introduced into a 2.5-l reactor. A batch process of anaerobic digestion was then applied using different hydraulic retention times (HRT), which ranged from 7 to 70 days. The temperature was set at approximately 35degreesC. Once the HRT was completed for every sample, a portion of the residue was classified under the Levine criterion (sedimentable, supracolloidals, colloidals and dissolved particles). This classification was formerly considered for municipal wastewater, although it has been also used for this kind of residue. Thus, the biomass evolution was followed along the process and correlated to a specific particle size fraction. The variation of the residue particle size, which was carried out with the method of series filtration, was linked to the system efficiency. As a result, this technique provides information about the process evolution, although the series filtration technique that was used is somewhat limited for this type of residue.