The kinetics of polycyclic aromatic hydrocarbons (PAH) elimination from a contaminated sludge were determined in bioreactors under different conditions: continuously oxic, anoxic, and anoxic/oxic oscillations. The dynamics of metabolically active bacterial communities and their involvement in PAH degradation were followed by T-RFLP targeting 16S rRNA and ring hydroxylating dioxygenase (RHD) transcripts, respectively. PAH degradation was related to toxicity elimination using an aryl hydrocarbon receptor-responsive reporter cell line. Oxygen supply was identified as the main factor affecting the structure of bacterial communities and PAH removal. PAH-degrading bacterial communities were stable throughout the experiment in all conditions according to the presence of RHD transcripts, indicating that bacterial communities were well adapted to the presence of pollutants. Oxic and anoxic/oxic oscillating conditions showed similar levels of PAH removal at the end of the experiment despite several anoxic periods in oscillating conditions. These results highlight the role of dioxygenase activity after oxygen addition. Nevertheless, the higher toxicity elimination observed under oxic conditions suggests that some metabolites or other unidentified active compounds persisted under oscillating and anoxic conditions. Our results emphasize the importance of using complementary biological, chemical and toxicological approaches to implement efficient bioremediation strategies. © Springer-Verlag 2012.