The granular sludges that were examined in this study are made up of a variety of methanogenic species, Methanothrix soehngenii being present in every sample. In general, their surface charge and specific suface area are compatible with a tightly packed structure having a low internal porosity. The stability of the granules depends on there being a polymer matrix at the outer surface although it is not known how this is formed. Calcium was not found to be present in significant concentrations except in one case. Silicon, phosphorus and sulphur were the more common elements. The non-granular sludge has an open lattice structure with the dominant bacterial species being filamentous, either Methanospirillum hungatei or Methanobacterium bryantii. The physical properties of this sludge were compatible with such a structure.