Fermentation of xylose by Clostridium thermosaccharolyticum was studied in batch and continuous culture in which the limiting nutrient was either xylose, phosphate, or ammonia. Transient results obtained in continuous cultures with batch grown inoculum and progressively higher feed substrate concentrations exhibited ethanol selectivities (moles ethanol/moles other products) in excess of 11. The hypothesis that this high ethanol selectivity was a general response to mineral nutrient limitation was tested but could not be supported. Growth and substrate consumption were related by the equation qs(1 − Y)GATP = (μ/Y) + m, with qs the specific rate of xylose consumption (moles xylose/hour · g cells), Y the carbon based cell yield (g cell carbon/g substrate carbon), GATP the ATP gain (moles ATP produces/mol substrate catabolized), μ the specific growth rate (1/h), Y the ATP-based cell yield (g cells/mol ATP), and m the maintenance coefficient (moles ATP/hour · g cells). Y was found to be 11.6 g cells/mol ATP, and m 9.3 mol ATP/hour · g cells for growth on defined medium. Different responses to nutrient limitation were observed depending on the mode of cultivation. Batch and immobilized cell continuous cultures decreased GATP by initiating production of the secondary metabolites, propanediol, and in some cases, D-lactate; in addition, batch cultures increased the fractional allocation of ATP to maintenance and/or wastage. Nitrogen-limited continuous free-cell cultures maintained a constant cell yield, whereas phosphate-limited continuous free-cell cultures did not. In the case of phosphate limitation, the decreased ATP demand associated with the lowered cell yield was accompanied by an increased rate of ATP consumption for maintenance and/or wastage. Neither nitrogen or phosphorus-limited continuous free-cell cultures exhibited an altered GATP in response to mineral nutrient limitation, and neither produced secondary metabolites. © 1993 John Wiley & Sons, Inc.