Aerobic granules to treat wastewater containing o-nitrophenol were successfully developed in a sequencing batch reactor (SBR) using activated sludge as inoculum. Stable aerobic granules were obtained with a clearly defined shape and diameters ranging from 2 to 6 mm after 122 days of operation. The integrity coefficient (IC) and granules density was found to be 98 and 1,054 kg m -3 respectively. After development of aerobic granules, o-nitrophenols were successfully degraded to an efficiency of 78 at a concentration of 70 mg L -1. GC-MS study revealed that the biodegradation of o-nitrophenol occured via catechol via nitrobenzene pathway. Specific o-nitrophenol biodegradation rates followed the Haldane model and the associated kinetic parameters were found as follows: V max = 3.96 g o-nitrophenol g -1VSS -1d -1, K s = 198.12 mg L -1, and K i = 31.16 mg L -1. The aerobic granules proved to be a feasible and effective way to degrade o-nitrophenol containing wastewater. © IWA Publishing 2012.