The growth of Thiobacillus ferrooxidans in a copper-containing ore suspension incubated in shake flasks was studied by determining the number of colony-forming units both in solution and attached to ore particles. The amounts of iron and copper released from the ore under experimental conditions were also determined. The total ferrous iron either released from the minerals or generated by reduction of the ferric iron in the minerals could account for the observed growth of bacteria in solution. Only a small fraction of the total colony-forming units-about 500 per mg ore-was found to be associated with the ore particles throughout the experiments. However, the rapid development of these colonies when ore particles were plated suggested that they were produced by a number of bacteria associated with each ore particle. Accordingly, when the amount of bacteria attached to ore particles was determined by monitoring the formation of ferric iron in the plates, the percentage of the total activity associated with attached bacteria was found to be between 1 and 10%.