In the course of investigation on the bioactive products of Alternaria porri (Ellis) Ciferri (the causal fungus of black spot disease in stone-leek and onion), previous studies reported three reduced anthraquinone derivatives (altersolanol A, B and dactylariol) and anthraquinone dimers (alterporriol A, B and C). This communication describes the isolation and identification of another bioactive compound, tentoxin, from A. porri. The fungus was cultured using stone-leek decoction medium for thirty days. Active substances were extracted with ethyl acetate from the culture liquid, concentrated, and subjected to preparative TLC (Merck Kieselgel 60PF254) with a solvent system of chloroform-acetone-formic acid (200:100:1). Repeated preparative TLC (Rf: 0.37) followed by recrystallization from benzene gave colorless needles (mp 173-174°C). Structural identification was based on spectral data: SIMS mass spectrum showed a peak at m/z 415 (MH+), molecular formula C22H30N4O4 (from El-high-resolution mass spectral data), IR spectrum (amide group at 1670, 1630 cm⁻¹; monosubstituted phenyl at 1520, 760, 700 cm⁻¹), and ¹H-NMR (400 MHz in CDCl₃) and ¹³C-NMR (INEPT method) spectra. Comparison with IR, NMR and mass spectra of an authentic sample confirmed the compound as tentoxin, a phytotoxin known to be produced by some Alternaria species (e.g., A. tenuis, A. mali, A. citri) causing chlorosis in dicotyledonous plants. From a chemotaxonomical perspective, it is of interest that tentoxin was identified as a metabolite of A. porri.