Escherichia coli KY10935, an L-methionine-auxotrophic mutant, has the ability to produce 100 g/l of L-threonine under limited conditions of DL-methionine, but occasionally exhibits overgrowth with a concomitant decrease in L-threonine production. We found that the growth of the producer depended not only on the availability of DL-methionine but also on that of iron, and that the dependence on iron caused abnormal fermentations in response to fluctuations in the iron concentration among lots of corn steep liquor, a cheap natural material used as the production medium. To overcome this problem, culture conditions were re-optimized with respect to the amounts of both DL-methionine and iron added in the medium so that the growth was controlled irrespective of the lot of corn steep liquor, to establish a stable industrial process with high productivity.