In the course of the screening for reacylation product of 4"-depropionyl maridomycin III (MDM III), Streptomyces sp. strain No. K-342 was found to reacylate it. The present study deals with the isolation and structure of the reacylation product designated as MDM K. Strain No. K-342 was cultured, and after adding 4"-depropionyl MDM III, MDM K was extracted from the culture filtrate with ethyl acetate, concentrated, and purified by silica gel column chromatography. To confirm MDM K as a reacylation product, the presence of an O-acyl mycarose moiety was ascertained via hydrolysis, ether extraction, chromatography, and IR spectroscopy, which showed the acyl group to be acetyl (identical to O-acetyl mycarose). Comparison of MDM K with MDM V revealed that their antibacterial activity (lower than MDM III but higher than 4"-depropionyl MDM III and similar to MDM V), molecular weight (815), thin-layer chromatogram Rf values, and spectral data (IR, Mass, NMR) were identical. Thus, MDM K is identical to MDM V, formed by introducing an acetyl group into the C4-position of the mycarose moiety of 4"-depropionyl MDM III.