BACKGROUND: Pyruvate decarboxylase (PDC) is a well-known pathway for ethanol production, but has not been demonstrated for high titer ethanol production at temperatures above 50 degrees C. RESULT: Here we examined the thermostability of eight PDCs. The purified bacterial enzymes retained 20% of activity after incubation for 30 min at 55 degrees C. Expression of these PDC genes, except the one from Zymomonas mobilis, improved ethanol production by Clostridium thermocellum. Ethanol production was further improved by expression of the heterologous alcohol dehydrogenase gene adhA from Thermoanaerobacterium saccharolyticum. CONCLUSION: The best PDC enzyme was from Acetobactor pasteurianus. A strain of C. thermocellum expressing the pdc gene from A. pasteurianus and the adhA gene from T. saccharolyticum was able to produce 21.3 g/L ethanol from 60 g/L cellulose, which is 70% of the theoretical maximum yield.