This study focuses on the decolorization of the C.I. Reactive Black 5 by a newly isolated yeast, Issatchenkia orientalis. The effect of varying initial dye concentration, inoculum concentration, carbon and nitrogen sources, pH, and NaCl was investigated. Maximum decolorization (99%) was achieved at initial dye concentration of 200 mg l^-1 within 24 h of incubation at 28 °C in the medium supplemented with 10% (v/v) inoculum concentration, 0.5% (w/v) glucose, and 0.1% (w/v) (NH₄)₂SO₄, and a wide range of pH values 3-8. Further experiment showed that this isolate also exhibited great color removal ability on other five dyes. The slightly colored biomass which was recovered after decolorization, and UV–vis spectra of supernatant suggested that both adsorption and biodegradation mechanisms may be contributed to the decolorization of C.I. Reactive Black 5. Moreover, a significant decrease in the activity of azoreductase in the cells after decolorization presumably indicated the involvement of this enzyme in the decolorization process.