An upflow anaerobic filter was developed with a microbial consortium, consisting predominantly of Alcaligenes faecalis and Commamonas acidovorans, immobilized on a gravel substratum. Remazol Black B, a commercially important textile dye, was decolourised by >95% within 48 h (operating conditions : initial dye concentration, 0.5g/l; pH 7.0; flow rate 0.1 1/hour; room temperature fluctuated between 12 and 20 degrees C).