A new antibiotic SF-733 inhibiting both Gram-negative and Gram-positive bacteria was isolated from Streptomyces ribosidificus nov. sp., strain SF-733. Antibiotic SF-733 obtained as colorless needles has a molecular formula C17H34N4O10,and is concluded to be a new aminoglycosidic, water-soluble and basic antibiotic based on its physical and chemical properties. In the course of a screening program for antibiotics active against Gram-negative bacteria, we found a new antibiotic, antibiotic SF-733, from the culture filtrate of a newly isolated streptomycete, strain SF-733 for which we propose the name Streptomyces ribosidificus nov. sp. Antibiotic SF-733 is a new amino-sugar antibiotic with low toxicity, and exhibits antibacterial activity against both Gram-negative and Gram-positive bacteria. Taxonomic studies on antibiotic SF-733-producing organism, and isolation and characterization of antibiotic SF-733 are reported in this paper.