Escherichia coli strain B, bearing the pet plasmid pLOI297, and the wild-type culture, lacking the plasmid, responded to aeration of the complex medium by an approximate three- and fourfold increase in both growth rate and growth yield with glucose and xylose, respectively. At a relatively low oxygen transfer rate (8 mmol O-2/L/h), the sugar-to-ethanol conversion efficiency exhibited by the recombinant strain decreased 40% and 30% for glucose and xylose, respectively. At a high aeration efficiency (100 mmol O-2/L/h), the ethanol yield with respect to xylose was 0.15 gig for the recombinant and 0.25 gig for the culture lacking the plasmid. These observations suggest that oxygen reduces the ethanologenic efficiency of recombinant E. coli by diverting carbon to growth and end products other than ethanol. Previous observations, by others, on the effect of oxygen on ethanogenic recombinant E. coli were made with different strains bearing different plasmids. In addition to the possibility of strain and plasmid specificity, the results of this study suggest that previous conclusions were influenced by the particular environmental conditons imposed on the culture, including poor aeration efficiency and lack of pH control.