Abstract: Dibenzofuran (DF) is one of the model compounds for the study of biodegradation of dioxin-like pollutants. This paper presents studies of the enhancing effect of a Janibacter strain on the degradation of DF by activated sludge and analyses of its DF-degrading genes. When 1% or 5 % Janibacter cells were added to the activated sludge reactor, DF of 56 mg/L was almost completely degraded after d8h or 24h, while in the control reactor, 38 % of the DF remained after 48h. A stretch of 4018-bp DNA region containing dfdA1A2A3A4 cluster was cloned with PCR and sequenced and found to be almost identical to that from Janibacter strain YK3, except 5 sbustitutions. To test whether the dfdA cluster is located on a large plasmid, we examined 27 single colonies grown on rich medium plates at 37℃ with PCR and found all tested colonies lost the dfdA cluster at high temperature. This result indicated that the gene cluster was readily cured from the cells, hence it must be located on an extra-chromosomal genetic element. Mutants abolished of the gene cluster were unable to grow in medium with DF as the sole carbon source. The implications of these results to the application of DF-degrading strains in activated sludge reactors are discussed.