A novel sulfur-containing compound named xanthienopyran was isolated from the fruits of Xanthium pungens (Compositae). The structure was determined by spectroscopic methods. In our previous paper, we characterized the structures of some sesquiterpenes in aerial parts of Xanthium pungens Wallr. mend. Widd. (Compositae). In continuation of our phytochemical research of Egyptian Compositae plants, we report here the isolation and structural elucidation of a novel sulfur-containing compound, xanthienopyran (1) in the fruits of X. pungens. Compound 1, a yellow amorphous powder, mp 218-220°C (dec.), [α]D²⁵ +7.91 (MeOH, c 0.48) was isolated from a dichloromethane-methanol extract of X. pungens fruits after successive column chromatography. The molecular formula of 1 was confirmed to be C15H16O4S by high-resolution EIMS which showed a molecular ion at m/z 316.0783 (calcd. 316.0765). The IR spectrum exhibited characteristic absorption bands at 3550-3100 and 1735 cm⁻¹ assignable to a hydroxyl and a carbonyl group. All 17 carbons were observed in the ¹³C NMR spectrum (acetone-d6) and the multiplicities were confirmed by DEPT and HMQC spectra. Among 16 protons in the ¹H NMR spectrum, 14 protons were fitted to the carbons by HMQC spectrum except for two protons assignable to a hydroxyl or a thionyl group. The structure of the side chain was elucidated by ¹H-¹H COSY spectrum. The chemical shift of C-14 (62.0 ppm) showed the presence of a hydroxyl group on the carbon. The stereochemistry of both C-9 and C-11 was E on the basis of J values (J9,10=15 Hz, J11,12=15 Hz). The structure of the chromophore was deduced from HMBC spectrum and NOE experiments as shown in Fig. 1. The chemical shifts of C-2 (144.6 ppm) and C-8b (127.1 ppm) compared with C-5 (150.8 ppm) or C-8 (81.5 ppm), which should bear an oxygen atom, fixed the position of the sulfur atom (S-1). Therefore, the structure of xanthienopyran was characterized as 1. The stereochemistry at C-8 could not be settled in the present study. According to the IUPAC system, the nomenclature of 1 is 5-hydroxy-8-(6-hydroxy-1E,3E-hexadienyl)-2-methylthieno[3',2':4,5]cyclopenta[1,2-c]pyran-6(8H)-one. The isolation and characterization of other compounds related to 1 in X. pungens is now in progress.