Aspergillus flavus oxidized elemental sulphur in vitro to thiosulphate, tetrathionate and sulphate, and mineralized peptone oxidizing the liberated ammonium to nitrite and nitrate. When elemental sulphur was added to peptone-containing medium the fungus oxidized sulphur but its ability to nitrify was inhibited. Such inhibition could have resulted from the reduction in medium pH following S-oxidation. Nitrification was still inhibited however, when S-oxidation occurred in medium buffered to pH 7.0, which suggests that pH was not the only factor inhibiting nitrification. Thiosulphate inhibited nitrification by A. flavus at concentrations similar to that produced by the fungus when oxidizing sulphur. Higher rates of nitrification by A. flavus occurred in medium containing ammonium sulphate and acetate, and in this medium thiosulphate (100 μg ml −1) prevented nitrification by A. flavus for one week, and reduced the rate of nitrification after three weeks. A. flavus appears to be incapable of simultaneously nitrifying and oxidizing sulphur due to the inhibitory effect of thiosulphate.