The objective of this work was to determine the effect of Leptospirillum ferrooxidans on the floatability of chalcopyrite, sphalerite, and pyrrhotite by using xanthate as a collector. The tests were carried out in the absence and presence of bacteria in relation to the type of ore and contact time with bacteria. The results indicate that the chalcopyrite flotation rate significantly increased in the presence of L ferrooxidans due to the formation of hydrophobic species. The bacteria function as a weak depressant for pyrrhotite after a conditioning time >= 60 min. The behaviour of sphalerite remains without changes due to its low susceptibility to oxidation. It was concluded that L ferrooxidans brings about superficial changes mainly due to the oxidation of minerals. (C) 2008 Elsevier Ltd. All rights reserved.