Performance of an anaerobic reactor in treating atrazine bearing wastewater using a mixed anaerobic microbial culture having high acetoclastic methanogen population was evaluated in batch and semi-continuous mode of operation. At an initial atrazine concentration of 5.0 mg/1, atrazine reduction of 30.2% after 5 days and 64% after 50 days were observed in semi continuous and batch mode of operation respectively. Carbonaceous material removal measured as chemical oxygen demand (COD) was more than 90% in both the cases. Initial COD ws 305±7 mg/1. Experimental results showed that atrazine degradation was better in presence of dextrose than that of sodium acetate. Percentage of methane in biogas was more in acetate fed reactor than dextrose fed reactor.