Ore samples from sulfidic mines in Romania were analyzed quant. for the occurrence of the ferrous-ion-oxidizing bacteria Thiobacillus ferrooxidans and Leptospirillum ferrooxidans. Thiobacillus ferrooxidans was detected regularly, whereas L. ferrooxidans occurred only at sites with a temp. above 20 DegC and with sufficient humidity. Accordingly, L. ferrooxidans survived preservation expts. only in nutrient soln. with and without pyrite for 56 days at 17 DegC. Thiobacillus ferrooxidans also survived this period of time in liq. nitrogen, frozen at -70 DegC with and without protective buffer, and after freeze-drying, although the cell nos. and activity were reduced. Thiobacillus thiooxidans survived all techniques. After growth on ferrous sulfate, the ferrous-ion-oxidizing activity of L. ferrooxidans was about 40% of the activity of T. ferrooxidans. After growth on sulfidic ore, both species exhibited a lower but similar activity. Cells of L. ferrooxidans exhibited 35% activity and cells of T. ferrooxidans 15%. Lab. expts. with pure cultures of these lithotrophs and mixed with strains of Acidiphilium sp. demonstrated that chemoorganotrophic microorganisms enhanced only the metabolic activity of L. ferrooxidans. Furthermore, in mixed culture with Acidiphilium sp., cells of L. ferrooxidans formed microcolonies or flocks. Flocks did not appear in pure cultures nor in mixed cultures of T. ferrooxidans and Acidiphilium strains. Considering the porous system of an ore particle in a humid environment, high ferric ion concn., and a low pH value, the results indicate that L. ferrooxidans is as important as T. ferrooxidans for metal solubilization and thus for microbial leaching.