We have recently shown (Lively et al., 1967) by the use of C14-labeled precursors that tryptophan, I, is a direct precursor of the antifungal substance pyrrolnitrin, II. Pyrrolnitrin, 3-chloro-4-(2'-nitro-3'-chlorophenyl)-pyrrole (Imanaka et al., 1965a; Nakano et al., 1966), is produced by several species of pseudomonads (Imanaka et al., 1965b); in our laboratory it is available from Pseudomonas aureofaciens ATCC 15926. This organism when grown on a complex or synthetic medium is capable of converting either endogenous or exogenous tryptophan to pyrrolnitrin. Thus, the addition of 500 µg/ml of dl-tryptophan to the culture mentioned resulted in increasing the yield of pyrrolnitrin from 50 to 100 µg/ml of broth. For the multi-step process, this represents a minimum yield of 8%, based on exogenous dl-tryptophan. We have now been able to add the tryptophan analogues, dl-6-fluorotryptophan (III) and dl-7-methyltryptophan (IV) to fermentations of P. aureofaciens and to recover the pyrrolnitrin derivatives, 3-chloro-4-(2'-nitro-3'-chloro-4'-fluorophenyl)-pyrrole (V) and 3-chloro-4-(2'-nitro-3'-methylphenyl)-pyrrole (VI), respectively. These new substances possess significant antifungal activity.