22 Mutants of Anabaena sphaerica 1017 resistant to 6-fluorotryptophan were isolated by spontaneous mutation and MNNG mutagensis. The mutagenic frequency was about 4 * 10~(-6), which was 40 times higher than the spontaneous frequency. The resistance was found to remain in the subculture on the presence of 6FT. These mutants were stable phenotypically, overproduce and excrete tryptophan to cross-feed the trp auxotroph of E.coli. Some of them can also alleviate the inhibition of 6FT for the growth of wild type on selective medium. The intracellular tryptophan content of the mutant (6FT-1) Was similar to that of wild type, but the tryptophan excreted by the mutant (6FT-1) was 2-3 times more than that of wild type. Addition of 6FT can promote 6FT-1 to excrete a large amount of tryptophan and some proteins. The extracellular aromatic amino acids inhibited the growth of Anabaena sphaerica 1017. Above results suggest other mechanisms which were not reported in cyanobacteria may present in Anabana sphaerica 1017, including the isoenzyme of tryptophan-specific DAHP Synthase in the biosynthesis of tryptophan.