In the present work, mutant strains of Klebsiella pneumoniae with deletions of the als gene encoding acetolactate synthase involved in synthesis of 2,3-butanediol, the ldhA gene encoding lactate dehydrogenase required for lactate synthesis, or both genes, were prepared. Production of 1,3-propanediol (1,3-PD) from glycerol was enhanced in the ldhA mutant strain (Delta ldhA), but lower in Delta als or Delta als Delta ldhA mutant strains compared to the parent strain, concomitant with a reduction in the glycerol consumption rate, indicating that deletion of ldhA alone was useful to improve 1,3-PD production. Fed-batch fermentation analysis revealed that, in the Delta ldhA mutant strain, 1,3-PD production was higher at low pH than at neutral pH; the reverse was true for the parent strain. Further optimization of culture conditions, by variation of aeration and glycerol feed rates, dramatically improved the production of 1,3-PD by the mutant strain. The maximum level attained was 102.7 g l(-1) of 1,3-PD from glycerol.