The leaching of iron pyrite by Thiobacillus ferrooxidans was studied in a continuous stirred tank reactor at a variety of dilution rates (0.012–0.22 h−1), pyrite surface areas (18–194 m2/L), and inlet soluble substrate (Fe2+) concentrations (0–3000 ppm). The bacterial leaching rate was found to increase with increasing pyrite surface area, dilution rate, and inlet Fe2+ concentration. The concentration of bacteria in solution was related to the concentration of bacteria attached to the pyrite surface by a Langmuir-type adsorption-desorption relation. Fitting the experimental data to this relation yielded a value for the area occupied per bacterium of 86 μm2. This result is consistent with the concept of preferential bacterial attachment of certain sites on the solid. A bacterial growth model was developed that included both bacterial growth in solution and growth of bacteria attached to the pyrite surface. The specific growth rate of the attached bacteria was calculated from this model and was found to increase with increasing solid dilution rate and to decrease with increasing pyrite surface area and soluble substance concentration. An explanation of these results based on an active-inactive site mechanisms was also developed.