Rosett, Sankhala, Stickings, Taylor & Thomas (1957) isolated tenuazonic acid (C₁₀H₁₅O₃N), a nitrogen-containing metabolite, from culture filtrates of Alternaria tenuis auct. grown on a glucose medium. This paper describes structural studies on tenuazonic acid: hydrolysis with 0.1 N mineral acid yields acetic acid and a colourless crystalline deacetyl compound (C₈H₁₀O₂N, m.p. 117.5–119.0°C); hydrolysis with 2 N mineral acid produces acetic acid, carbon dioxide, and a primary amine (3-amino-4-methylhexan-2-one, identified via its benzenesulphonyl derivative). Ozonolysis in aqueous alkali followed by acid hydrolysis yields acetic acid, oxalic acid, and L-isoleucine (confirmed by elemental analysis, optical rotation, paper chromatography, and ninhydrin degradation to (+)-2-methylbutanal 2:4-dinitrophenylhydrazone). Chemical properties (strong monobasic acidity with pK ~3.35, intense ferric reaction, complex copper salt formation) indicate an enolic hydroxyl group α to carbonyl groups and an acetyl group at position 3. Tenuazonic acid is formulated as α-acetyl-γ-sec-butyltetramic acid, which explains its reactivity (e.g., ozonolysis, acid hydrolysis) and aligns with its similarity to α-acyltetronic acids.