A strain of Escherichia coli was engineered to overproduce L-tryptophan. A fed-batch fermentation process was developed, producing 30.8+/-1.4g dm(-3) with a yield on glucose of 0.132+/-0.010g g(-1). Specific production rate did not appear to be limited by cloned enzyme activity, but by the carbon flux from central metabolism into the aromatic amino acid pathway. The glucose feed rate profile was modified in an attempt to increase the production rate. Tryptophan production was not affected, but led to glutamic acid excretion at high levels. The high specific glucose consumption rate at the low growth rate led to the high glutamate excretion. A new fermentation process involving modification of the feed profile to limit the formation of by-products was discovered. The resulting final process increased tryptophan production to 42.3+/-2.7g dm(-3) with yield on glucose of 0.176+/-0.006g g(-1). The instantaneous yield realized the theoretical maximum for the majority of the fermentation.