A novel spiro-monoterpene-5-methylcoumarin, named spiro-ethuliacoumarin, was isolated from the aerial parts of Erhulia conyzoides. The structure was determined by spectroscopic methods and X-ray crystallography. In continuation of our studies (A.A.A. and A.A.M.) on the terpenoid constituents of Egyptian compositaeous plants, we have isolated a spiro-monoterpene-5-methylcoumarin (1) from Erhulia conyzoides L. (Compositae). Several structurally close relatives were isolated from E. conyzoides, 1-4 and recently an unusual [2+2] cyclo-adduct of monoterpenoid-5-methylcoumarin has been also isolated from E. conyzoides. Although the monoterpenoid part is usually connected to an oxygen at C-3 or C-4 in the monoterpenoid-5-methylcoumarin series, 1 has a different connection from these monoterpenoid-coumarins, and then a new mode of connection is proposed here. Compound 1 (spiro-ethuliacoumarin), colorless needles, mp 198°C, [α]D +167.39 (CHCl3, c 1.515) was isolated from an ether fraction of the aerial parts of E. conyzoides extracted with Et2O-petrol-MeOH. The high resolution electron-impact MS showed a molecular ion at m/z 340.1290 (calculated 340.1310 for C20H20O5). The IR spectrum displayed characteristic absorption bands for C=O of lactone ring (1760 cm⁻¹) and α,β-unsaturated ketone (1680 cm⁻¹). The ¹H-NMR spectrum (Table 1) showed one-proton aromatic signals at δ 7.09 (d, J=8 Hz), 7.48 (dd, J=8, 8 Hz), and 7.00 (d, J=8 Hz) and a three-proton singlet at δ 2.62, which are possibly assignable to hydrogens of a 5-methylcoumarin. The ¹³C-NMR spectrum (Table 1) exhibited twenty carbon signals which correspond to the molecular formula composed of a 5-methylcoumarin (C₁₀) and a monoterpene (C₁₀). A partial structure of the monoterpene moiety was elucidated as follows: the presence of a 7',9'-dihydro-5',8'-γ-lactone ring was inferred from the secondary methyl group signal at δ 1.04 (3H, d, J=7.5 Hz) (H-9') which showed a coupling with a double quartet centered at δ 2.93 (1H, J=11, 7.5 Hz) (H-7') in the ¹H-¹H COSY spectrum, and from a resonance of a carbonyl carbon at δ 177.9 ascribed to C=O of the γ-lactone in the ¹³C-NMR spectrum. Furthermore, one oxygen-bearing methine proton signal centered at δ 5.09 (1H, ddd, J=8, 7.8, 5.4 Hz) assignable to H-5' in the ¹H-NMR spectrum was correlated with a carbon signal at δ 78.4 in the ¹H-¹³C COSY spectrum, and the methine proton also showed a coupling with a proton signals at δ 4.51 (1H, dd, J=11, 8 Hz) assignable to H-6' which was coupled with H-7' (δ 2.93) in the ¹H-¹H COSY spectrum. These accumulated data suggested the presence of the partial structure of a Me-CH-CH-CH(OR)-CH₂- in the monoterpene moiety. In addition, the down-field proton signals at δ 5.51 (1H, dd, J=17, 9 Hz), 4.94 (1H, d, J=9 Hz) and 4.92 (1H, d, J=17 Hz) showed correlations with carbon resonances at δ 137.8 (d) and 117.8 (t) in the ¹H-¹³C COSY spectrum, indicating unequivocally the existence of a vinyl group. The assignment of the carbons in 1 was achieved by ¹H-¹³C COSY except for quaternary carbons at δ 56.4 and 70.8, and carbonyl carbons at δ 167.4, 177.9 and 191.0. The COLOC experiments (Table 1) enabled the connection between all sequences of both the monoterpenoid and the 5-methylcoumarin parts through the quaternary carbons, the carbonyl carbons and the tertiary methyl groups. The most important correlations were observed between H-6' and C-2, C-3, C-4, C-7', C-8', and between each H-7', H-10' and C-3. All these results proved that 1 has a spiro moiety between the monoterpene and the 5-methylcoumarin parts and they are fused at C-3 through the central carbon. The stereochemistry of 1 was confirmed by the NOE experiments (Fig. 1). The final proof of the complete structure and the relative stereochemistry were established by the X-ray analysis of 1 (Fig. 2).