Abstract: This invention is concerned with the production of L-tryptophan by strains of Escherichia coli.The invention provides a host and method for growing that host such that plasmids can be stably maintained in the absence of antibiotics, leading to high productivity of tryptophan in fermentation. It offers significant improvements over previous methods and produces tryptophan free from any contamination by tyrosine an phenylalanine thus simplifying downstream processing.The E. coli strains having productivity for L-tryptophan comprise in their genome a mutant gene encoding a partially defective tryptophanyl-tRNA synthetase introducing a temperature-conditional tryptophan auxotrophy in the host and further mutations on the chromosome which disable the transport systems encoded by aroP, mtr and tnaB and being transformed with a plasmid containing a tryptophan operon encoding anthranilate synthase freed from feedback-inhibition by tryptophan.