Pyrrolnitrin is an antifungal antibiotic and is most active against dermatophytic fungi, especially the species of Trichophyton. Fist, physical and chemical properties of pyrrolnitrin was described by Arima et al. It is synthesized from tryptophan, and the synthesis is probably initiated by a chloroperoxidase enzyme system acting on tryptophan. The present report deals with the mechanism of action of pyrrolnitrin. Pseudomonas pyrrocinia and other Pseudomonas species produce pyrrolnitrin from tryptophan as secondary metabolite. Pyrrolnitrin inhibits fungal growth by inhibiting the respiratory electron transport system.