The essential oils isolated at the flowering stage in spring, summer and autumn of the aerial and underground parts of Conyza bonariensis L. growing in Tunisia were analysed by GC and GC/MS; 143 constituents were identified, and among them 20 were major. The oils of C. bonariensis gathered in spring were rich in matricaria ester (1.2–67.3%), (Z)-nerolidol (0.3–19.9%) and caryophyllene oxide (0.8–14.3%). In the summer samples, the oils of C. bonariensis were rich in matricaria ester (1.6–76.4%), caryophyllene oxide (1.6–22.6%) and (E)---farnesene (1.1–22.7%). The main constituents in the autumn samples were matricaria ester (trace to 63.5%), geranyl acetone (0.0–25.3%), trans--bergamotene (0.0–24.3%) and limonene (2.7–15.3%). The oils showed significant variation among the seasons. Remarkable differences were found between the constituent percentages of the different studied organs. The C. bonariensis sample from Tunisia was a matricaria ester chemotype and was quite different from the samples of other countries of origin.