Crystalline minerals in anaerobically digested sewage sludges were determined by x‐ray diffraction analysis. Sludge samples were prepared for x‐ray analysis by either washing with H2O to remove soluble salts or oxidizing organic matter with H2O. A limited number of minerals are present as crystalline materials in sludge. Even though the sludges contained appreciable concentrations of Cu, Zn, Cd, Pb and Ni, no crystalline metal sulfides, phosphates, hydroxides, oxides or carbonates were found with the exception of a possible Cu, Zn carbonate hydroxide. All other crystalline components detected are common minerals such as quartz, feldspar, montmorillonite, chlorite, mica, dolomite and calcite.