Sulfobacillus thermosulfidooxidans cultures adapted to ferrous or tetrathionate ions were inoculated into media containing both substrates. In batch culture, cells showed a preference for oxidising ferrous ion followed by tetrathionate. Tetrathionate oxidation was slower in ferrous-adapted cells. Biomass formation exhibited an exponential growth phase during ferrous ion oxidation followed by an exponential growth phase during tetrathionate oxidation. Sequential utilisation of ferrous ion followed by tetrathionate ion was observed when equimolar amounts of substrates or electron-equivalent amounts were provided.