Iron-oxidizing thiobacilli oxidize many sulfide minerals. It has been suggested that one mechanism of the oxidation involves a mineral surface attack by the bacteria, thereby catalyzing chemical reactions that normally proceed at slow rates. Evidence for a direct surface attack by acidophilic thiobacilli has been obtained from scanning electron microscope studies in which organisms have been shown to colonize mineral surfaces, eventually resulting in pitting and corrosion of the substratum. Scanning electron microscopic characterization of bacterial-mineral interactions is limited in that the technique is not readily suitable to estimate the kinetics of the bacterial sorption to particles. In a search for suitable methods for evaluating bacterial sorption, autoradiographic techniques have been investigated; similarly, measurements of biomass have been used to examine the time course of bacterial sorption. In the present work, cell protein was used as a measure of monitoring the sorption of Thiobacillus ferrooxidans to various fine-grain particulates. The materials chosen for the study include both oxidizable substrates and inert particles that were not chemically altered during their exposure to the bacteria suspensions. In the present communication, the term ''sorption'' is used without regard to the electrochemical properties, charge effects, and biological mechanism of attachment, all of which influencemore » the bacterial sorption on solids.« less