For the purpose of studying the production of L-tryptophan by Escherichia coli, the deletion mutants of the trp operon (trpAE1) were transformed with mutant plasmids carrying the trp operon whose anthranilate synthase and phosphoribosyl anthranilate transferase (anthranilate aggregate), respectively, had been desensitized to tryptophan inhibition. In addition to release of the anthranilate aggregate from the feedback inhibition required for plasmids such as pSC101 trp.I15, the properties of trp repression (trpR) and tryptophanase deficiency (tnaA) were both indispensable for host strains such as strain Tna (trpAE1 trpR tnaA). The gene dosage effects on tryptophan synthase activities and on production of tryptophan were assessed. A moderate plasmid copy number, approximately five per chromosome, was optimal for tryptophan production. Similarly, an appropriate release of the anthranilate aggregate from feedback inhibition was also a necessary step to ward off the metabolic anomaly. If the mutant plasmid pSC101 trp-I15 was further mutagenized (pSC101 trp.I15.14) and then transferred to Tna cells, an effective enhancement of tryptophan production was achieved. Although further improvement of the host-plasmid system is needed before commercial production of tryptophan can be realized by this means, a promising step toward this goal has been established.