Microthrix parvicella can cause poor settlement or foam problems in the activated-sludge process. It is normally identified on the basis of its morphology and its staining behaviour. However, it has never been characterised in terms of its biochemical reactions or its genetic pattern. This paper reports the results of a series of biochemical tests on M. parvicella that had been isolated from activated sludge. Bacillus subtilis and three nocardioforms were also tested. A comparison of the results showed that the profile for M. parvicella was different from that of other organisms and also from that of other species that had been examined in earlier studies.