The degradation of an azo dye, Orange II, by immobilised Phanerochaete chrysosporium in a continuous packed bed bioreactor for periods longer than 30 days has been carried out. Nearly complete decolourisation (>95%) was achieved when working at a high dye load rate of 0.2 g x l(-1) x d(-1), a temperature of 37 degrees C, a hydraulic retention time (HRT) of 24 h and applying oxygen gas in a pulsed flow. These conditions allowed Manganese peroxidase (MnP) production and the subsequently Orange II decolourisation. A correlation between residual MnP activity in the effluent and decolourisation was established. Apparently, for decolourisation to be effective, a minimum MnP activity was required, no substantial increase in efficiency at MnP activities higher than 10 U x 1(-1) was observed. The treatment caused, the breakdown of the chromophoric group as well as the cleavage of the aromatic ring.