The stability in aqueous suspensions of two particulate celluloses, Sigmacell type 100 and Avicel PH101, was analyzed. The effect of the presence of a cellulase from Trichoderma reesei, ionic strength, and ultrasonic agitation on the fragmentation/aggregation phenomena was studied. Particle size distributions of the powders were obtained with three different particle sizers : the Galai CIS 100, the Coulter Multisizer II, and the Malvern 2600c. The differences in the obtained absolute values are discussed according to the measuring principles of each technique; however, the overall conclusions are independent of the particle sizer used. The enzyme breaks up the Avicel aggregates more effectively than ultrasound while the Sigmacell particles are stable under the present experimental conditions. The stabilizing effect of celluloses was tentatively explained using the DLVO (Derjaguin, Landau, Verweye, and Overbeek) theory. The adsorbed enzyme did not change significantly the zeta potential of the fibers, hence, the stabilizing effect was attributed to a reduction in the attractive van der Waals forces and hydration effects.