Studies on the microbiology of anaerobic sludge disclosed that non-methanogenic strictly anaerobic bacteria comprised one of the most numerous groups of organisms present. These previously unreported organisms are mainly un-named Bacteroides species. Euryoxic bacteria constitute only a minor fraction of the normal bacterial flora but can become predominant in very short time if fermentable carbohydrates are present. This finding emphasizes the necessity of using unenriched fermentations for studying the sludge digestion process. Work is in progress to determine the source of substrates and the organisms responsible for the generation of acetic acid (the main methane-producing substrate). (Author)