Batch and continuous studies have been conducted on the treatment of black Liquor from a kraft pulp and paper mill by a bacterial strain, Aeromonas formicans. The results of batch studies revealed that the strain was able to remove 71% and 78% of COD and lignin respectively, while the colour removal efficiency was around 86% in 10 days of retention time. The analysis of Lignin degradation products by gas chromatography after 20 days of incubation revealed the formation of some phenolic acids, which were responsible for the decrease in pH during batch studies. The removal efficiencies of GOD, colour and Lignin obtained in continuous reactor studies were 73, 88 and 77% respectively for an 8 day detention period and these efficiencies were almost the same as obtained in batch studies.