Although over 1,500 halogenated natural products have been identified in animals, plants, and microorganisms,1'^only two biological routes for their formation are known: the direct methylation of halide ions,3) and haloperoxidase-catalyzed halogenation.2?3) The biosynthesis of armentomycin (L-2-amino-4,4-dichlorobutanoic acid, 6), a nonprotein amino acid with antibiotic properties40 is of particular interest because no mechanism is known for the chloroperoxidase-catalyzed insertion of chlorine substituents remote from other functional groups.2) Wepresent here the initial results from our investigation of armentomycin biosynthesis in Streptomycesarmentosusvar. armentosus. A starch, L-lysine and KC1 medium (100ml/500-ml Erlenmeyer flask) was inoculated with a 48-hour seed culture of S. armentosus var. armentosus UC2862and incubated at 27°C with rotary shaking (220rpm).5) Research Council of Canada (NSERC), and by an exchange fellowship (to J.-Y. H.) from the World University Services of Canada. Weare indebted to the Atlantic Region Magnetic Resonance Centre for providing NMRspectra.