Abstract: A filamentous fungi was isolated from the biofilter of a freshwater aquacultural system and identified as Aspergillus niger NBG5, which can consume ammonium nitrogen, at rates of 0.247gN(gcell)−1 day−1, at 30°C. A high concentration favors removal of nitrogenous wastes. In this investigation, A. niger NBG5 was employed in a continuously stirred tank reactor (CSTR) system to remove nitrogen. A total ammonia nitrogen (TAN) of 25mgL−1 was removed within 35h from artificial wastewater. TAN and nitrite–N concentrations were decreased to 0.35 and 0.12mgL−1, respectively, passing CSTR at a hydraulic retention time (HRT) of 3.3h. [Copyright &y& Elsevier]