Atrazine degradation by anaerobic mixed culture microorganism in co-metabolic process and in absence of external carbon and nitrogen source was studied at influent atrazine concentration range of 0.5-15mg/l. Wastewater of desired characteristic was prepared by the addition of various constituents in distilled water spiked with atrazine. In co-metabolic process, dextrose of various concentrations (150-2000mg/l) was supplied as external carbon source. The reactors were operated in sequential batch mode in which 20% of treated effluent was replaced by the same amount of fresh wastewater everyday, thus maintaining a hydraulic retention time (HRT) equal to 5 days. In co-metabolic process, 40-50% of influent atrazine degradation was observed. First-order atrazine degradation rate (expressed in day-1) was better in co-metabolic process (5.5×10-4) than in absence of external carbon source (2.5×10-5) or carbon and nitrogen source (1.67×10-5). In presence of 2000mg/l of dextrose, atrazine degradation was between 8% and 15% only. Maximum atrazine degradation was observed from wastewater containing 300mg/l of dextrose and 5mg/l of atrazine. Influent atrazine concentration did not have much effect on the methanogenic bacteria which was clear from methane gas production and specific methanogenic activity (SMA). © 2004 Elsevier Ltd. All rights reserved.