We reinvestigated the aerial parts of Pluchea dioscoridis (L.) DC. [=Convza dioscoridis (L.) desf.], which grows wild in Egypt, to see whether there is a seasonal variation in the chemical constituents. This work afforded three new compounds, i.e., 15-hydroxyisocostic acid (Ia), the corresponding aldehyde (Ie) and the eudesmanolide (IIa). Other known products were also isolated, including the thiopheneacetylenes (IIIa, b), eudesmanolides (IV, V) and 9β-hydroxycostunolide (VI). 15-Hydroxyisocostic acid Ia and the corresponding Ie are rather polar, and hence their separation was difficult. However, it was achieved after diazomethane treatment. Thus, Ia was transformed to the methyl ester Ic as well as to the 15-O-methyl ether Id. Compound Ic gave no molecular ion, but the observed fragment m/z 246, corresponding to C₁₆H₂₂O₂, was clearly formed by loss of water from the parent ion. The presence of a hydroxyl group was indicated by the infrared (IR) spectrum (3615 cm⁻¹) and also by the oxidation of Ia with manganese dioxide to give an aldehydic product (Ib). The latter compound was identical with that obtained by esterification of the corresponding naturally occurring acid Ie. The proton nuclear magnetic resonance (¹H-NMR) (400 MHz) spectrum of Ic was in part similar to that of the known 15-desoxy derivative, but the signal of the olefinic methyl group was replaced by a pair of doublets at δ = 4.09 and 3.02. These signals were replaced by a singlet at δ = 10.17, after oxidation of Ic. Thus, the structures of Ib and Ic were settled and the natural products were identified as Ia and Ie. The structure of IIa was deduced from the ¹H-NMR spectrum, which displayed signals at δ 3.08 br d (H-5), 4.05 t (H-6) and 3.28 ddddd (H-7). The signal of H-1 appeared at 3.89 br d (J=4 Hz), which differed from that of IIb in the changed couplings of H-1, indicating the presence of the epimer. Further evidence for structure IIa was obtained from the mass and IR spectra. The seasonal difference in the constituents of P. dioscoridis is worthy of comment. The sample collected in August from the same locality did not contain any isocostic acid derivatives, while the sesquiterpene lactones and other constituents were nearly the same. Only II was not present, though 1α-acyloxy derivatives with a 3.4-epoxide group (Va, b) were isolated. The isolation of these further sesquiterpenes clearly supports the replacement of Pluchea in the Inulinae since this type of sesquiterpene is widespread among other genera of this subtribe.