Finely ground particles (pyrite ore, uranium‐bearing quartzite, fluorapatite, sea sand, pulverized quartz, and glass beads) retarded iron oxidation in cultures ofThiobacillus ferrooxidansgrowing at pH 1.5. The inhibition was dependent on the shaking of the culture flasks and was less pronounced in static cultures. Experiments with finely ground particles indicated that the inhibition was related to the concentration and size of the particles present in the culture solution. Fluorapatite was anomalous because below 2% (w/v) it completely prevented iron oxidation but had little or no effect at higher concentrations.