This paper deals with the bio-oxidation of galena particles(-80 meshes) using Acidithiobacillus ferrooxidans and compares it with Fe3+ oxidation.Experimental results show that,at least,0.00197 mol galena was leached from 100 mL pulp(density of 3.8%) with 39 days’ bio-oxidation,as compared to 0.00329 mol galena by Fe3+ with 9 days’ oxidation.Because Fe3+ was constantly consumed,leaching by Fe3+ almost stopped after 9 days.Large amounts of lead sulfate were detected in both bio-oxidation and Fe3+ oxidation of galena.A.ferrooxidans followed a unique growth pattern during the bio-oxidation of galena.In the initial 15 days,the bacteria attached themselves to the galena surface with the formation of erosion pits similar in shape and length to those of the bacteria,and there were hardly any bacteria suspended in the solution.After 15 days,suspended bacteria increased.It is thus suggested that A.ferrooxidans may directly oxidize galena. This paper deals with the bio-oxidation of galena particles (-80 meshes) using Acidithiobacillus ferrooxidans and compares it with Fe3 + oxidation. Experimental results show that, at least, 0.00197 mol galena was leached from 100 mL pulp (density of 3.8%) with 39 days ’bio-oxidation, as compared to 0.00329 mol galena by Fe3 + with 9 days’ oxidation.Because Fe3 + was of consumed, leaching by Fe3 + almost stopped after 9 days. Larger amounts of lead sulfate were detected in both bio-oxidation and Fe3 + oxidation of galena.A.ferrooxidans followed a unique growth pattern during the bio-oxidation of galena.In the initial 15 days, the bacteria attached themselves to the galena surface with the formation of erosion pits similar in shape and length to those of the bacteria , and there were hardly any bacteria suspended in the solution. After 15 days, suspended bacteria increased. It is thus suggested that A. ferrooxidans may directly oxidize galena.