Several filamentous bacteria obtained in pure culture from activated sludge plants were characterised using their 16S rDNA sequences to determine their phylogenetic relationship to other bacteria. “Microthrix parvicella” was shown to be an unusual actinomycete, while the Gram negative bacteria Type 0092, Type 0411 and Type 1863 all belong to the Flexibacter-Cytophaga-Bacteroides phylum, and Type 0803 is a member of the beta subclass of the Proteobacteria. The practical value of obtaining this information is discussed.