Two obligately anaerobic bacterial strains were isolated from the contents of a pilot scale, anaerobic digester treating slaughterhouse waste with a high protein and lipid content. The isolates, LIP1 and MW8, were characterized as spore-forming, Gram-positive rods, capable of fermenting glycerol. Isolate LIP1 was also observed to be lipolytic and was able to hydrolyse tallow and olive oil. Both isolates grew optimally at 37 °C and formed either acetate and formate (LIP1), or acetate and butyrate (MW8), as major glycerol fermentation products. Both isolates produced ethanol as the major reduced fermentation end-product. Neither MW8 nor LIP1 had growth and metabolism inhibited by the addition of stearic acid at concentrations normally considered bactericidal. Analysis of the 16S rRNA gene sequences, in conjunction with the phenotypic data, confirmed that the isolates are members of the genus Clostridium (sensu lato), clustering with species of clostridial clusters I (MW8) and XIVa (LIP1).