In this study, the acute toxicities of nitrobenzene (NB) and para nitrophenol (p-NP) were investigated in a high rate sequential anaerobic migrating blanket (AMBR)/aerobic completely stirred tank reactor (CSTR) using Microtox and Daphnia magna tests. After sequential anaerobic and aerobic treatments, the inhibitions in the Microtox bacteria decreased from an initial 78.10-48.20% and 4.00%, respectively, in wastewater containing 40.00 mg/Lp-NP. The inhibitions of the influent wastewater containing 60.00 mg/L NB decreased from 72.10% to 45.30% and to 4.00% after anaerobic and aerobic treatment, respectively. The acute toxicity removals were 94% and 93% in the effluent of the whole sequential system, for p-NP and NB, respectively. The acute toxicity in the influent was dependent on the parent NB and p-NP concentrations and ons their physicochemical properties such as hydrophobicity, octanol/water partition coefficient and vapour density for both Microtox bacteria and Daphnia magna while the toxicity in the effluent of the anaerobic reactor was strongly dependent on the metabolites of p-NP (p-amino phenol, phenol, NH4-N) and NB (aniline) for Microtox test. This effluent was not toxic to Daphnia magna. (C) 2010 Elsevier B.V. All rights reserved.