In this study the anaerobic treatability of p-nitrophenol (p-NP) was investigated in an anaerobic/aerobic sequential reactor system under increasing organic loadings during operation period of 186 days. Lab-scale anaerobic migrating blanket reactor (AMBR)/completely stirred tank reactor (CSTR) system consisted of effective volumes of 13.5 and 9 L, respectively. Synthetic wastewater was obtained from constant p-NP concentration of 40 mg/L and a glucose concentration giving 3000 mg/L COD as primary substrate. The studies were carried out in continuous mode and the effluent of the AMBR reactor was used as the feed for the CSTR reactor. The AMBR/CSTR reactor system was operated at organic loading rates (OLRs) increasing from 0.31 to 3.25 kg/m(3) day. COD and p-NP removal efficiencies were decreased from 92% to 79% and from 94% to 86%, day in AMBR. The total COD and p-NP removal efficiencies respectively, when the COD loading rates were increased from 0.31 to 3.25 kg/m(3) were 94% and 91% in whole (AMBR/CSTR) reactor system, at a COD loading rate of 3.25 ka/m(3) day, respectively. The total gas and methane gas production increased with increased OLRs. The daily total and methane gas productions were 12.25 and 3.8 L/day at OLR of 3.25 kg/m(3) day, respectively. Methane gas percentage was also found as 31% at this loading rate. In this study, pH, total volatile fatty acid (TVFA) and bicarbonate alkalinity (BA)/total volatile fatty acid (TVFA) ratios variations in the effluent and in the all compartment of the AMBR were suitable for optimal anaerobic treatment at all OLRs. The pH, TVFA and BA/TVFA ratios in the effluent and in all the compartments of the AMBR were obtained between 7.2-7.64, 290-353 mg/L and 0.06-0.13, respectively, at OLR of 3.25 kg/m(3) day. A 79-92% of the p-NP transformed top-amino phenol (p-AP) in anaerobic phase. The generation of phenol and ammonia (NH4-N) was observed during the anaerobic degradation. A complete mineralization of p-AP (E = 100%) was found in the aerobic phase. The p-AP, phenol and NH4-N were converted to nitrite (NO2-N), nitrate (NO3-N) and CO2 as end products in the aerobic step. (c) 2006 Published by Elsevier Inc.