Ethanol fermentation under non-sterilized condition was developed in this paper. An acid-tolerant mutant of Zymomonas mobilis, which can begin fermentation at the low pH of 4.5, was selected. At this low pH, potential contamination of ethanol production was controlled at the normal level without autoclaving or filter sterilization. Compared with the filter sterilization process, the same ethanol yield of 73.1 g/l indicated that no contamination occurred in the non-sterilized process. Compared with the autoclaved process, ethanol yield increased from 70.3 to 73.2 g/l and residual sugar concentration decreased from 5.3 to 1.3 g/l with 40 h fermentation. The theoretical yield of ethanol from glucose was 0.488 g/g in this new ethanol process. (C) 2004 Elsevier Ltd. All rights reserved.