Recent literature has showed the potentiality of microorganisms as a flotation reagent. In the present study, a non-pathogenic hydrophobic bacterium (Rhodococcus opacus) has been investigated as a flotation reagent for the hematite-quartz system. The studies were based on electrophoretic mobility and contact angle measurements, scanning electron microscopy (SEM) and microflotation experiments with and without prior biotreatment. Microbe-mineral interactions resulted in significant surface-chemical changes on mineral surfaces. The microflotation tests with mineral mixtures showed that through biotreatment, it becomes possible to float the hematite and depress the quartz particles. The changes in electrokinetic behavior contact angles and flotation behavior of quartz particles after microbial interaction are supported by DLVO theory for the pH range evaluated. For hematite, this theory was suitable to predict the adhesion only for a restricted pH range. (C) 2003 Published by Elsevier Science B.V.