During a screening program for new antibiotics, a novel nucleoside antibiotic (1) was isolated from the cultured broth of Serratia plymuthica BMI585-mF4. The strain was fermented in a production medium composed of mannitol, yeast extract, (NH₄)₂SO₄, KCl, K₂HPO₄, and CaCO₃ at pH 7.4, followed by purification via sequential steps including Diaion HP-20 adsorption, silica gel 60 chromatography, Sephadex LH-20 chromatography, and Diaion CHP-20 linear gradient elution. Physicochemical properties (e.g., white powder appearance, SI-MS [m/z 303 (MH⁺)], FD-MS [m/z 302 (M⁺)], molecular formula C₁₁H₁₄N₂O₈, optical rotation [α]ᴰ -2.4° (c 0.5, H₂O), UV, IR), nuclear magnetic resonance (¹H/¹³C NMR, HMBC) analysis, and mild acid hydrolysis (yielding 5-hydroxymethyluridine confirmed by comparison with an authentic sample) revealed the structure of 1 as 5-formyloxymethyluridine. The antibiotic showed weak activity against some Gram-negative bacteria, with minimum inhibitory concentrations (MICs) against various bacteria listed in Table 3. Intravenous injection of 50 mg/kg to mice did not exhibit any toxic signs.