The mode of attachment of dimethylallyl pyrophosphate (DMAPP) in the biosynthesis of the indole alkaloids paraherquamide A, austamide, and brevianamide A has been studied. Feeding experiments or, Penicillium fellutanum, Penicillium brevicompactum, and Aspergillus ustus using [C-13(2)]-acetate showed isotopic scrambling of the geminal methyl groups originating from C-2 of the indole ring precursors in paraherquamide A, brevianamide A, and austamide biosynthesis. The labeling patterns suggest that the methyl groups of dimethylallyl pyrophosphate become equivalent during the biosyntheses; a non-face-selective, S-N' mechanism has been invoked to account for these observations.