The effects of pretreatment by overliming and addition of nutrients (yeast extract, tryptone and ammonium chloride) on fermentation of mixed sugars derived by acid hydrolysis of the hemicellulose fraction of wheat straw with Bacillus stearothermophilus strain T-13, an L-lactate dehydrogenase deficient mutant was investigated in continuous culture with partial cell recycle. Pretreatment and addition of nutrients to the hydrolysate improved the fermentation considerably. Sugar utilization, ethanol yields and productivities obtained in the treated hydrolysate with added nutrients were comparable to those obtained in a synthetic medium. Sugar utilization in the synthetic medium and treated and crude hydrolysates with added nutrients were 86%, 89% and 56%, respectively, compared with 45% in the treated hydrolysate without extra nutrients. Ethanol yields obtained were 0.32 g g(-1) sugars and 0.38 g g(-1) sugars in the treated hydrolysate with and without extra nutrients, respectively, compared with 0.24 g g(-1) sugars in the crude hydrolysate with added nutrients. Continuous culture with partial cell recycle significantly increased the rate of ethanol production (0.60-1.02 g litre(-1) h(-1)) in the various media and the stability of the mutant strain compared with conventional continuous culture.