In this study, the anaerobic treatability of p-nitrophenol (p-NP) was investigated in an anaerobic/aerobic sequential reactor system. During the anaerobic stage the target compound is modified via degradation and then the intermediate product is more readily biodegradable by the aerobic phase. Lab-scale anaerobic migrating blanket reactor (AMBR) and a completely stirred tank reactor (CSTR) were used sequentially to investigate the treatability of p-nitrophenol using glucose as primary substrate in continuous fed mode. The COD removal efficiency of synthetic wastewater containing no p-NP was between 90 and 95% after 45 days of the start-up period. The effects of increasing p-NP concentrations on bicarbonate alkalinity, volatile fatty acid accumulation, and methane gas percentage were obtained in AMBR reactor while, NO2-N, NO3-N, NH4-N were monitored as final metabolic products in aerobic CSTR reactor. COD and p-NP removal efficiencies of 94 and 95%, were obtained respectively, in AMBR reactor at a loading rate of 3.85 g p-NP/m(3)/day. The maximum methane production rate and methane percentage (approximately 1000 ml/day and 50%, respectively) was obtained at this loading rate. p-NP was transformed to p-aminophenol (p-AP) in the anaerobic phase. Mineralization of p-AP and phenol was determined in the oxidative aerobic stage. The total p-NP and COD removal efficiencies were found as 96 and 97% at loading rate of 3.85 g p-NP/m(3)/day in whole (anaerobic/aerobic) sequential reactor system, respectively. (c) 2004 Elsevier Ltd. All rights reserved.