The production of volatile industrial chemicals utilizing metabolically engineered extreme thermophiles offers the potential for processes with simultaneous fermentation and product separation. An excellent target chemical for such a process is acetone (T-b = 56 degrees C), ideally produced from lignocellulosic biomass.Caldicellulosiruptor bescii(T(opt)78 degrees C), an extremely thermophilic fermentative bacterium naturally capable of deconstructing and fermenting lignocellulose, was metabolically engineered to produce acetone. When the acetone pathway construct was integrated into a parent strain containing the bifunctional alcohol dehydrogenase fromClostridium thermocellum, acetone was produced at 9.1 mM (0.53 g/L), in addition to minimal ethanol 3.3 mM (0.15 g/L), along with net acetate consumption. This demonstrates thatC. besciican be engineered with balanced pathways in which renewable carbohydrate sources are converted to useful metabolites, primarily acetone and H-2, without net production of its native fermentation products, acetate and lactate.