This study investigates the effect of hypochloride application for controlling bulking and foaming on the microbial ecology of an activated sludge system treating tannery wastewater. Detailed characterization of the wastewater treatment influent and effluent is also reported for the study period. During the study, bulking and foaming are first monitored with a sudden burst in the sludge volume index over 250 mL g(-1), creating a significant deterioration of the effluent quality. The corresponding upset in the microbial ecology is the combined excessive proliferation of M. parvicella, N. limicola II and Gordona (Nocardia) spp., but mainly triggered by Gordona contamination of the floc structure and the rapid outward growth of this filamentous microorganism extending to adjacent flocs. Chlorine application at an average rate of 3 g Cl-1(kg MLSS.day)(-1) for 12 days provide an effective solution for bulking and foaming, restoring the effluent quality. It destroys filamentous texture between the flocs, leaving only a lot of loose and chopped filament fragments and, totally removes the Gordona spp. from solution which retrieves back into the flocs. Therefore, chlorine remediation of bulking and foaming, although temporarily effective, is only superficial as the Gordona seeding inside the floc remains intact and potentially available for excessive growth in the next favorable conditions.