Thiobacillus ferrooxidans was grown in shake flask cultures, using powdered sulphur as the energy source. Growth of cultures was followed by monitoring the pH of the medium. Low levels (10ppm) of soluble silver nitrate (AgNO3) were introduced to some cultures, where it was absorbed by the bacteria. Silver absorption resulted in a lag time in the growth phase, but not a decrease in the subsequent growth rate, and a change in the colour of the culture from bright sulphur yellow to dark grey. Zeta potentials for both these silver-loaded bacteria and a control culture were measured using a particle microelectrophoretic technique. The zeta potentials of the control bacteria were found to be consistent with those recorded by other workers, with an IEP at pH 2.4. Zeta potentials of silver loaded bacteria were found to be very different exhibiting an IEP below pH 2.0, a minimum zeta potential of -32mV at pH 7.0, and an increase in zeta potential at higher pH conditions. The zeta potential of pure silver sulphide was also measured, and found to have an IEP similar to that of the control culture. Between the IEP and pH 5, the zeta potential of the silver loaded bacteria is close to the sum of the potentials of the control bacteri and the pure mineral. It is postulated that the enhanced zeta potential is due to the additional surface potential of the silver sulphide precipitates. The results raise the possibility of selective recovery of silver-absorbing bacteria, and a novel silver recovery system.