In this study, the anaerobic treatability of high strength opium alkaloids processing industry wastewaters was investigated. The wastewater was fed to a lab-scale anaerobic upflow sludge blanket reactor (UASBR) operating at mesophilic conditions. The UASB reactor (11.5 L) was operated for 825 days at different hydraulic retention times ranging from 0.85 to 1.62 days and at organic loading rates ranging from 3.40 to 12.25 kg COD/m(3). day. Anaerobic treatability studies indicated that 300 L CH(4) can be produced per kg COD removed. At the 445th day of the study an extreme irreversible inhibition caused by N,N-dimethylaniline was experienced. This paper demonstrated that opium alkaloid industry wastewater can be efficiently treated by UASB type reactors with more than 80% COD removal efficiency at high organic loading rates.