A strain of streptomycete was found to produce a new antibiotic pyrroxamycin. This compoundwas isolated from the culture broth of Streptomyces sp. S46506. The chemical structure was determined to be 4>5-dichloro-2-(6/,8/-dichloro-4/Jfir-r,3/-benzodioxin-4/-yl)-3nitropyrrole by its chemical character and XHand 13C NMRspectral analysis. Pyrroxamycin was active against Gram-positivebacteria and dermatophytes. In the course of our screening program for new antibiotics, a streptomycete, strain S46506, designated as Streptomyces sp. S46506 was found to produce a new antibiotic named pyrroxamycin (1). This compound was previously called SS46506A substance1}. The chemical structure of pyrroxamycin, a new antibiotic related to pyrrolomycins, was determined to be 4,5-dichloro-2-(6',8/-dichloro-4'iif-r, S'-benzodioxm-^-yO-S-nitropyrrole by its chemical character and XHand 13C NMRspectral analysis. Pyrroxamycin was active against Gram-positive bacteria and dermatophytes. This paper deals with taxonomy of producing strain, fermentation, isolation, physico-chemical properties, chemical structure and biological properties of pyrroxamycin. Taxonomy Strain S46506 was isolated from a soil sample collected at Kamagaya City, Chiba Prefecture, Japan. The mediumused for actinomycete isolation was tomato juice-oatmeal agar composed of oatmeal 20 g, tomato juice (centrifuged supernatant, 10,000 rpm, 10 minutes) 100 ml, distilled water 900 ml, Bacto agar (Difco) 20 g, nystatin 40 mg, cycloheximide 40 mg and novobiocin 10 mg. The strain has been deposited at The Fermentation Research Institute, Agency of Industrial Science and Technology, Japan, where it has been assigned accession number FERMP-8020. Taxonomic studies were carried out according to the methods described by Shirling and Gottlieb2) and of Waksman3). The color notations were from the ISCC-NBSCentroid Color Charts1. The inoculation of strain S46506 onto various media was carried out using the washed cells cultured in yeaststarch broth at 28°C for 5 days on a reciprocal shaker. Morphological observations were made of the cultures grown at 28°C for 7~21 days on thin potato-carrot agar and water agar4). Mature spore chains had 10 or more spores in the form of spirals (Fig. 1). Spirals were observed in spots on the above media. The spores were oval or cylindrical and 0.5~0.8 x0.8~1.2^min size with a smooth surface under an electron microscope (Fig. 2). No sporangia or flagellated spores were observed.