A new antibiotic, 6"'-O-a-n-mannopyranosyl mannosidostreptomycin has been isolated from the culture filtrate of strain MD753-C2, together with streptomycin and mannosidostreptomycin. This strain has been classified as Streptomyces sioyaensis1) which is a new streptomycin-producer. In this communication, the isolation, characterization and structural elucidation of the antibiotic are reported.