Biological oxidation of a refractory arsenic-bearing gold ore was investigated to determine the fate of arsenic during processing and to assess the effect of biological pretreatment on gold extraction. Experiments were conducted using a consortium of iron and sulfide oxidizing bacteria in semi-batch and continuous-flow stirred bioreactors, and also in a tall column bioleaching system. Sulfide oxidation and arsenic solubilization were monitored during biooxidation and the data were correlated with precious metals extraction during subsequent cyanidation tests. In addition, the concentrations of arsenite [As(III)] and arsenate [As(V)] species present in the liquors were monitored as a function of time using an anion exchange high performance liquid chromatography/atomic adsorption spectroscopy technique. The stabilities of the final tailings and the arsenic-bearing sludges obtained by neutralization of the biooxidation liquors were also assessed. Leaching results showed that over 95% sulfide oxidation and 47% arsenic extraction could be obtained from this ore using the biooxidation process. Gold extraction during subsequent cyanidation was increased from 49% to about 80% after biological pretreatment. Total arsenic concentration as high as 17 g/L in solution were obtained with no apparent detrimental effect on the bacterial populations. In all cases, arsenic was initially solubilized as As(III). Typical leaching solutions contained up to 18 times as much as As(III) as As(V). Since As(III) compounds are less stable and more more toxic than As(V) compounds, long long-term sludge materials in the environment is of concern. Results from Toxicity Characteristic Leaching Procedure stability tests performed on the sludges will be discussed.