Treating soluble wastewaters anaerobically in UASB-reactors, the development of sludge-granules can be expected, provided that there are optimal growing conditions. Two types of granules were obtained, both with high specific activities and excellent settling properties. Type 1 mainly consists of short fragments of multicellular filaments of a bacterium showing much resemblance with a recently isolated acetate-degrading methanogenic bacterium (Zehnder et al, 1980). Type 2 is mainly made up by long filaments of presumably the same bacterium. The development of anaerobic bulking sludge must be prevented.