This study investigates the anaerobic treatability of Reactive Black 5 in an anaerobic/aerobic sequential process. Laboratory scale upflow anaerobic sludge blanket (UASB) reactor/completely stirred tank reactors (CSTR) were operated at different organic loadings and hydraulic retention times (HRT). The effects of shock organic loading on the chemical oxygen demand (COD), color removal, and methane gas production efficiencies were investigated in UASB reactor. The effects of both sludge retention time (SRT) and food to mass (F/M) ratio on the color and COD removal efficiencies were also investigated in aerobic reactor. The reactive dye used in this study contains the groups N=N, -SO3, S=O and is in use in the some textile industries in Turkey. The studies were carried out in continuous mode and the effluent of the UASB reactor was used as feed for the CSTR reactor. COD removal efficiencies decreased from 56 to 27% with increases in COD loadings from 5 to 25 kg COD m(-3) per day in the anaerobic UASB reactor. The color removals were 92 and 87%, respectively, for aforementioned organic loadings. The methane percentages were found to be 76 and 60% at organic loading rates of 2.49 and 14.8 kg COD m(-3) per day, respectively, in UASB reactor. COD removal efficiencies of 28, 42, and 90% were obtained at SRTs of 1.7, 5.7 and I I days in the aerobic CSTR reactor. Optimum sludge retention time was 11 days in aerobic reactor. A total of 67 and 28% COD removal efficiencies were obtained at F/M ratios varying between 0.05 and 0.17 and between 0.30 and 1.4 kg COD kg(-1) MLSS per day. A 90-95% color and 40-60% COD removal efficiencies were obtained depending on the applied organic loadings in the UASB reactor. The remaining COD was removed with a treatment efficiency of 85-90% in the aerobic CSTR reactor.