The objective of this work was to evaluate the post-treatment of an anaerobic recalcitrant effluent (anaerobically-treated weak black liquor, AnE) in an aerobic, upflow reactor packed with "biocubes" of Trametes uersicolor immobilized onto small cubes of holm oak wood. The treated effluent (named anaerobic effluent; AnE) from all anaerobic fluidized bed reactor was fed to an up-flow aerobic fungal packed bed reactor (PBR). Two HRT were tested in this unit, namely 5 and 2.5 days; the PBR operated 60 days at 5-day HRT and 35 days at 2.5-day HRT. The aerobic packed bench scale reactor was a glass column 1.5 L total geometric volume containing 0.75 L biocubes of T. versicolor immobilized onto holm oak wood small cubes of 5 mm side. The reactor was operated at 25 C. The pH of the AnE was adjusted to 4.5 before feeding; no carbohydrates or other soluble carbon source was supplemented. The fungal packed bed bioreactor averaged organic matter removals of 30% and 32%, COD basis, during an experimental run of 60 days at 5-day HRT and 35 days at 2.5-day HRT, respectively. Colour and ligninoids contents were removed at higher percentages (69%, and 54% respectively, average of both HRT). There was no significant difference between reactor performance at 5- and 2.5-day HRT, so, operation at 2.5-day HRT is recommended since reactor throughput is double. Activity of manganese peroxidase and laccase was found during the entire operation of the fungal PBR whereas lignin peroxidase activity practically disappeared in the second operation period. In general, enzyme activities were higher in the first period of operation (5-day HRT) than at 2.5-day HRT. To the best of our knowledge, this is one of the few works that demonstrated extended performance (3 months) of a fungal bioreactor for the treatment of a recalcitrant wastewater with no supplementation of glucose or other expensive, soluble carbohydrate. (c) 2007 Published by Elsevier Ltd.