A bacterial consortium that can degrade chloro- and nitrophenols has been isolated from the rhizosphere of Phragmitis communis. Degradation of 4-chlorophenol (4-CP) by a consortium attached to granular activated carbon (GAC) in a biofilm reactor was evaluated during both open and closed modes of operation. During the operation of the biofilm reactor, 4-CP was not detected in the column effluent, being either adsorbed to the GAC or biodegraded by the consortium. When 4-CP at 100 mg l(-1) was fed to the column in open mode operation (20 mg g(-1) GAC total supply), up to 27% was immediately available for biodegradation, the rest being adsorbed to the GAG. Biodegradation continued after the system was returned to closed mode operation, indicating that GAC bound 4-CP became available to the consortium. Biofilm batch cultures supplied with 10-216 mg 4-CP g(-1) GAC suggested that a residual fraction of GAG-bound 4-CP was biologically unavailable. The consortium was able to metabolise 4-CP after perturbations by the addition of chromium (Cr VI) at 1-5 mg l(-1) and nitrate at concentrations up to 400 mg l(-1). The development of the biofilm structure was analysed by scanning electron microscopy and confocal laser scanning microscopy (CLSM) techniques. CLSM revealed a heterogeneous structure with a network of channels throughout the biofilm, partially occupied by microbial exopolymer structures.