The decolorization of solutions containing the textile reactive dye Orange II by the bacteria Geobacillus stearothermophilus (UCP 986) was investigated. The experiments were statistically evaluated by two-level-four factor (2 ⁴) full factorial design, as aeration, azo dye concentration, inoculum size, and culture media. The best results were with Luria Bertani (LB) medium under aeration, and the decolorization efficiency meant that 96—98% of the azo dye was removed after 24 h of incubation. The efficiency of LB medium in color reduction was due to a co-metabolic process. Sulphanilic acid (4-aminobenzene sulphonic acid) was not detected. The testing for toxicity using the brine shrimp Artemia salina showed the absence of toxic metabolites during the decolorization process; however, when the concentration of the dye and agitation were high (0.050 mM and 150 rpm) the shrimp showed high mortality rates (LC₅₀ (lethal dose — assay concentration able to cause 50% of death) 49.28% v/v). The parameters such as culture medium and agitation are significantly important to the decolorization of the Orange II process. G. stearothermophilus (UCP 986) showed a high biotechnology potential for textile bioremediation of azo dye under the co-metabolic process.