Recently, a gene for a 7-dimethylallyltryptophan synthase (7-DMATS) was identified in Aspergillus fumigatus and its enzymatic function was proven biochemically. In this study, the behaviour of 7-DMATS towards aromatic substrates was investigated and compared with that of the 4-dimethylallyltryptophan synthase FgaPT2 from the same fungus. In total, 24 simple indole derivatives were tested as potential substrates for 7-DMATS. With an exception of 7-methyltryptophan, all of the substances were accepted by 7-DMATS and converted to their prenylated derivatives, indicating a more flexible substrate specificity of 7-DMATS in comparison to that of FgaPT2. The relative activities of 7-DMATS towards these substrates were from 4% to 89% of that of L-tryptophan, much higher than that of FgaPT2. Structural elucidation of the isolated enzymatic products by nuclear magnetic resonance and mass spectrometry analysis proved unequivocally the prenylation at position C7 of the indole ring. Overnight incubation with eight substances showed that the conversion ratios were in the range of 55.9% to 99.7%. This study provided an additional example that prenylated indole derivatives can be effectively produced by using the overproduced and purified 7-DMATS.