Aspergillus niger oxidized the sulphides of copper, lead and zinc, but not cadmium, to sulphate. With the exception of cadmium sulphide, the sulphide particles in the medium were adsorbed on to the surface of the mycelium, a process which is apparently associated with sulphide oxidation. Traces of thiosulphate and tetrathionate were also produced, and the medium was acidified. Smaller amounts of sulphate were found in sulphide-amended media in which Trichoderma harzianum grew compared with media supporting A. niger, although thiosulphate and tetrathionate were produced more consistently. As the amounts of sulphate taken up by the two fungi could not be accurately determined, it is unclear whether these differences in sulphate concentration in the media indicate differences in rates of sulphide oxidation, or merely reflect differing rates of sulphate assimilation. Again, with the exception of cadmium the concentration of free metal in the medium did not generally increase, presumably because any metal released from the sulphides on oxidation was taken up by the fungi, or adsorbed on to their surface. It is unlikely therefore that these fungi could be effectively employed to leach metals from their sulphide ores.