A tryptophan auxotrophic strain named Tyr2 was screened out by treating wild E. coli ym1013 with NTG.After Tyr2 was further irradiated by UV,a strain ZC6 which could grow in media containing 8 mg/ mL P-FPA,was obtained. Fermentation indicated that the yield of phenylalanine by the mutant strain ZC6 was 6 times of that by wild type strain ym1013.