In this communication we report the isolation and X-ray crystal structure of an ichthyotoxic metabolite of G. breve. The dinoflagellate was cultured in 15-L solution bottles (10-L volume) in NH-15 medium. The cells were exposed to a 24-h light period, 500 foot-candles. The cells were harvested at the peak of the growth period by direct extraction with chloroform. The residue from the chloroform extract was successively extracted with hexane, benzene, and chloroform. The chloroform-soluble fraction after repeated HPLC gave a crystalline compound (I) which was recrystallized with benzene to give colorless needles: mp 82-83 °C; IR (CCl₄) νmax 3590, 3350, 2930 cm⁻¹; C₁₀H₂₂N₃O₃PS [by high-resolution mass spectrometry (calcd. 295.1119, found 295.1135) and by X-ray analysis]. X-ray analysis was carried out on crystals from benzene. Thin, plate-shaped crystals of the compound belong to the monoclinic space group P2₁/c, with a = 11.255 (4) Å, b = 17.694 (9) Å, c = 8.214 (2) Å, β = 109.60 (2)°, V = 1541.0 ų, Z = 4, at -135 (2) °C. The structure was determined by a combination of direct methods, Patterson analyses, and difference Fourier syntheses. The compound was thus identified as O,O-dipropyl (E)-2-(1-methyl-2-oxopropylidene)phosphorohydrazidothioate (E)-oxime (Figure 1), C₁₀H₂₂N₃O₃PS. The 400-MHz ¹H NMR spectrum (CDCl₃) showed the presence of two methyl groups on vinylic carbon(s) and two symmetrical n-propyl groups. Although the presence of phosphorus-containing toxic metabolites has been reported, this is the first report on the structure of a phosphorus-containing compound from G. breve. Compound I (tentatively named Gb-4) has an ichthyotoxicity of 0.9 ppm against Lebistes reticulatus. The crude extract of G. breve has at least five more toxic compounds including Brevetoxin B. Work on the structure elucidation of the other ichthyotoxic compounds is in progress.