Electrobioleaching of a sphalerite flotation concentrate at a positive DC potential of +0.4V (SCE) has been discussed and compared with that at -0.5V(SCE). Probable mechanisms in the electrobioleaching process are considered with respect to oxidative and reductive dissolution as well as to direct and indirect roles of Thiobacillus ferrooxidans. The effect of applied potentials on mineral dissolution and bacterial activity is illustrated.