During the researches on thermophylic fungi our attention was drawn to a strain belonging to Aspergillus flavipes series (1) which demonstrated a strong activity against gram-positive and gram-negative bacteria. The strain F-2091/7 of our collection was grown in submerged culture in Erlenmeyer flasks of 500 ml capacity containing 100 ml of a modification of the nutrient medium reported by Stoll and coworkers (2). The antibiotic production began at 24°C after 3 days culture, reaching the highest activity after 7 days with a maximum concentration of about 300 pg/ml (measured in agar plate using a strain of B. subtilis as test organism). The antibiotic was extracted from the fermentation broth acidified to pH 3.5 with chloroform, purified by chromatography on acid alumina (C. Erba) using CHCl3:Et2O=4:1 as eluent, and recrystallized from benzene to give the new antibiotic (I) as optically active ([α]D²⁰= -71.80; C=1% in 95% ethanol) white needles with m.p. 130-131°C. Elemental analysis gave results consistent with a crude formula C12H15NO4, confirmed by mass spectrometry (found M+=237.100). Structural analysis using UV, IR, NMR spectra, and catalytic hydrogenation experiments revealed that (I) belongs to the cycloheximide family, although this is the first time that a member of this family is found in fungi. At the contrary of cycloheximide, this new antibiotic, as shown in Table No. 1, is active against gram-positive and gram-negative bacteria and inactive against fungi. Studies on possible therapeutic applications are in program.