Fortimicins and sporaricins represent a new class of aminoglycoside consisting of a pseudodisaccharide and an amino acid. We report a newcomer to this class, substance SF-2052 (I), which is the first aminoglycoside having a formimino moiety. The antibiotic-producing organism, strain SF-2052, was isolated from soil collected at Matsuzaki-cho, Izu Peninsula, Japan, and designated as Dactylosporangium matsuzakiense sp. nov. (a rare actinomycete forming finger-shaped sporangia with a single row of three motile spores and no aerial mycelium). Substance SF-2052 was produced by submerged cultivation of Dactylosporangium matsuzakiense SF-2052 in a 300-liter jar fermentor (medium containing starch, peptone, wheat germ, etc.; 28°C for 4 days) with a maximum titer of 80 μg/ml (paper disc method using Bacillus subtilis ATCC 6633). The fermented broth was purified via acid filtration, Amberlite IRC-50 adsorption, HCl elution, active carbon treatment, CM-Sephadex C-25 chromatography, etc., to obtain SF-2052 sulfate (3.45 g). SF-2052 sulfate is a white powder, melting at 176-178°C with decomposition, soluble in water, less soluble in methanol, almost insoluble in acetone and ethyl acetate; it shows positive ninhydrin, GREIG-LEABACK and LEMIEUX reactions, negative SAKAGUCHI reaction; its spectral data include [α]D +81° (c 1.0, water), UV end absorption, IR peaks at 3400, 1720, 1643, 1520, 1110 and 1060 cm⁻¹, PMR signals (e.g., 1.33 (3H d, C-CH₃), 3.16 (3H s, N-CH₃), 7.98 (1H s, CH-N)), and CMR data. The molecular formula C₁₈H₃₆N₆O₆•2H₂SO₄•H₂O was confirmed by CMR carbon count and elemental analysis. The structure of SF-2052 (I) was determined by hydrolysis (acid hydrolysis yielded glycine and formiminoglycine; alkaline hydrolysis afforded fortimicin B) and CMR similarity to fortimicin A. SF-2052 showed in vitro activity against Gram-positive and Gram-negative bacteria (including aminoglycoside-resistant organisms) and in vivo activity against Escherichia coli, Klebsiella pneumoniae, Serratia marcescens, Pseudomonas aeruginosa (ED₅₀ values listed). The acute LD₅₀ of SF-2052 sulfate in mice was 350 mg/kg by intravenous route. These results suggest SF-2052 is a useful antibiotic. Details of the taxonomic study of strain SF-2052, structure determination and evaluation will be described separately.