The effect of temperature and substrate concentration (TOC) on the growth of Thermus thermophilus cells in batch fermentation were investigated, and the data were correlated using unstructured specific growth rate equations. The batch fermentations showed low substrate utilization at about 35% of the initial total organic carbon content (TOC) of the substrate. Intermittent substrate feeding strategies were adopted at two low substrate concentrations The fed-batch fermentations which were conducted to maintain relatively low constant substrate concentrations in the medium did not improve substrate utilization, but led to biomass production comparable to that obtained in simple batch fermentations with the same total amount of substrates. The optimal induction time for D-glucose isomerase activity by D-xylose was found to he at the end of the exponential phase.