The isolation of a lysine and an arginine auxotrophic mutant from the asexual yeast Candida utilis 189 was achieved by a combination of benomyl treatment, mutagenesis with ethyl methane sulphonate and mutant enrichment using nystatin. The amino acid auxotrophic mutants showed an increased ability to convert anthranilic acid to L-tryptophan in response to starvation for the required amino acids in the growth medium. An improved strain (arg- 2-2), which accumulated 9.2 g litre-1 L-tryptophan within 5 days of fermentation was isolated from the arginine auxotroph on media containing 5-fluorotryptophan and anthranilic acid.