Effect of continuous feeding of a reactive azo dye, reactive orange 107, and its hydrolyzed form (HRO107), on a conventional anaerobic digester was investigated in this study together with observation of change in microbial community. Laboratory-scale digesters were fed with waste activated sludge and azo dye for 575 days continuously. The influent concentrations of reactive azo dye were between 200 and 3200 mg/L. The digester performance was not adversely affected by azo dye and its reduction metabolites throughout the study. A HRO107 concentration with 3200 mg/L was not inhibitory for the digester performance after prolonged exposure. The dye removal efficiencies were between 95 and 99% most of the time of operation implying that dye removal efficiency was not affected by variations in organic loading rate or influent dye concentration. Sulfate-reducing bacteria may increase in the digester but did not cause any toxicity due to hydrogen sulfide formation after prolonged feeding.