The importance of L. ferrooxidans for leach processes was evaluated by studying the lithotrophic flora of 3 mine biotopes and a heap leaching operation, by percolation expts. with inoculated, sterilized ore, and by morphol., physiol., and genetic investigations of pure and mixed cultures of L. ferrooxidans, Thiobacillus ferroxidans, and Thiobacillus thiooxidans. In biotopes of >=20 Deg, Leptospirillum-like bacteria are as abundant as T. ferrooxidans. Leptospirilli represent at >=1/2 of the Fe2+-oxidizing population. Percolation expts. confirmed this result. Leptospirilli were as numerous as T. ferrooxidans. At reduced temps., the generation times of leptospirilli increase more so than those of T. ferrooxidans. At 14 Deg, Leptospirillum grows slowly and T. ferrooxidans dominates the population. Physiol. investigations indicate that L. ferrooxidans is a strict chemolithoautotroph, metabolizing only Fe2+ and pyrite. Even an addn. of 0.05% yeast ext. inhibited its growth. The max. Fe2+-oxidizing activity of L. ferrooxidans amts. to .apprx.40% of the activity of T. ferrooxidans. After growth on sulfidic ore, both spp. exhibit reduced Fe-oxidizing activities, L. ferrooxidans exhibiting 1/3 and T. ferrooxidans exhibiting 1/7 of their max. activities. Surprisingly, the abs. values are similar. For indirect leaching, L. ferrooxidans is as important as T. ferrooxidans. This was confirmed by the results of percolation expts. L. ferrooxidans together with T. ferrooxidans mobilized metals at least as well as T. ferrooxidans did. The best results were obtained with a mixed culture of all 3 spp.