Zymomonas mobilis ZM4/Ac-R (pZB5), a mutant recombinant strain with increased acetate resistance, has been isolated following electroporation of Z. mobilis ZM4/Ac-R. This mutant strain showed enhanced kinetic characteristics in the presence of 12 g sodium acetate l(-1) at pH 5 in batch culture on 40 g glucose, 40 g xylose l(-1) medium when compared to ZM4 (pZB5). In continuous culture, there was evidence of increased maintenance energy requirements/uncoupling of metabolism for ZM4/Ac-R (pZB5) in the presence of sodium acetate; a result confirmed by analysis of the effect of acetate on other strains of Z. mobilis.