Some physical properties of several digested sludges have been measured to better characterize the sludge particle system. The particle size distributions of two sludges were measured with a Coulter Counter. Number counts were in excess of 1012/1 and the majority of the particles had diameters less than 5 mm. Particle surface areas were at least 20 m2/1. Particle sizing by filtration was attempted but proved unsuccessful due to membrane poor clogging. Sedimentation experiments for sludge in artificial seawater showed decreasing sedimentation velocities with increasing dilution (and hence lower particle number concentration). A comparison of sedimentation velocities for four sludges from different sources showed that sedimentation velocities increased with increasing solids content. Data presented suggest flocculation of the sludges in seawater increased the sedimentation velocities.