Gencor Limited are pioneers in the commercialization of the bio-oxidation of refractory gold ores. They started developing the bio-oxidation (BIOX process in the late 1970s at Gencor Process Research, and the process is now being used in five com. plants. The paper describes the BIOX plants at Fairview (South Africa), Sao Bento (Brazil), Harbor Lights and Wiluna (Australia), and Sansu (Ghana), giving details of the tests conducted and the results achieved. A sixth BIOX plant is to be constructed in Peru for the Proano Tamboraque project. The plant will treat 60 t of high-arsenic concentrate per day and should be commissioned towards the end of 1996. During the BIOX process, arsenic and sulfur are solubilized before being neutralized with limestone and/or milk of lime in a two-stage process. Results from the five operating plants, as well as from extensive laboratory and pilot-scale testwork, have proved that the basic ferric arsenate precipitate formed during neutralization is stable when the molar ratio of Fe3+ to As5+ exceeds 3:1. The capital cost of a BIOX plant is significantly lower than that of a pressure-oxidation plant of similar capacity. The operating cost compares favorably with that of competing processes.