An unique up-flow anaerobic stage reactor (UASR) treating pharmaceutical wastewater at various organic loading rate (OLR) was investigated and showed efficient substrate removal at low OLRs (0.43-1.86 kg COD m(-3)d(-1)) by promoting efficient chemical oxygen demand (COD) reduction (70-75%). However, increasing the OLRs to 3.73 kg COD m(-3) d(-1) by reducing the hydraulic retention time (HRT) (4-2 d) reduced :he COD removal efficiency (45%). Fluorescent in situ hybridisation (FISH) analysis showed that the microbial community of the reactor stages was dominated by methanogenic archaea (56-79% of 4',6 diamidino-2-phenylindole (DAPI)-stained cells in all stages) when the reactor was fed at OLR 0.86-2.98 kg COD m(-3)d(-1) but decreased in Stage 1 (30% of DAPI-stained cells) when the OLR was at 3.73 kg COD m(-3)d(-1). The two methanogenic genera detected were Methanosarcina and Methanosaeta. Generally, Methanosaeta dominated the reactor stages when the OLR was 0.86-1.86 kg COD m(-3) d(-1), while Methanosarcina dominated during the period of high OLR (2.98-3.73 kg COD m(-3)d(-1)). Based on the number of Desulfovibrio cells detected (8-36% of the total eubacteria) in all stages during the entire operation of the reactor, it appears that sulfidogenic bacteria contributed to the overall degradation of organic substances in the pharmaceutical wastewater. (C) 2010 Elsevier B.V. All rights reserved.