Although current bio-oxidation processes with mesophilic bacteria result from the occurrence of mixed populations, the mutual effect of the various species has not been studied very extensively to date. Mixed cultures made up of pure Thiobacillus ferrooxidans, Thiobacillus thiooxidans and Leptospirillum ferrooxidans strains of the DSM collection were batch tested for their ability to oxidize a cobaltiferous pyrite ore. The most efficient population for pyrite oxidation was composed of the three bacterial species. The influence of the relative abundance of the different strains in the inoculum was studied. The cobalt solubilization rate obtained with T. ferrooxidans increased when L. ferrooxidans was present but was not affected by the initial concentration of L. ferrooxidans. The bioleaching with T. ferrooxidans was only improved by adding T. thiooxidans when the initial concentration of T. thiooxidans was higher than the initial concentration of T. ferrooxidans. During continuous bioleaching of the cobaltiferous pyrite at 20% solids with a natural mesophilic mixed population, rod-shaped and Leptospirillum-like bacteria were enumerated in the liquid phase. The 100 l bioleaching unit is made up of 3 or 4 reactors arranged in cascade. The concentration of Leptospirillum-like organisms rose exponentially versus dissolved ferric iron, whereas the concentration of rod-shaped. bacteria did not change from the value obtained in the first reactor, providing the solution contained less than 60 g.l(-1) ferric iron. At higher Fe3+ concentrations, the rod-shaped population in the liquid phase decreased. These results suggest that the rod-shaped bacteria performed the earlier steps of pyrite oxidation, whereas Leptospirillum-like organisms participated in the later phase of bioleaching. The effluent from the last reactor was treated with CaCO3 in order to precipitate iron. Recycling of partially neutralized bioleach solution to the feed seemed to increase the concentration of Leptospirillum-like bacteria in all reactors. Two hypotheses are proposed to explain this phenomenon: some bacteria may have been brought into the first tank with the recycled liquid, or the cobalt concentration may have affected the distribution of Leptospirillum-like bacteria between liquid and solid phases.