A hyperbolic equation was fitted to the time dependence of Thiobacillus ferrooxidans adsorption on sulfur. This procedure allowed the determination of an asymptote value which represented a free cell count at equilibrium. The data thus obtained were used to calculate the numbers of adsorbed bacteria without attainment of equilibrium. The maximum number of adsorbed bacteria based on the Langmuir isotherm was 8.3 x 10(9) per g of sulfur.