Sweet fennel (Foeniculum vulgare Mill. var. dulce Mil) is a major aromatic plant belonging to the Apiaceae. The study was conducted to determine variations of plant properties (fruit yield per plant, 1000-fruit weight), essential oil content and composition during four different maturation stages (immature, premature, mature and full mature) in sweet fennel fruit. Essential oil obtained by hydro-distillation was characterized by gas chromatography (GC) and gas chromatography/mass spectrometry (GC-MS). Fruit yield per plant and 1000-fruit weight regularly increased from immature to mature periods, while essential oil content declined with fruit maturity. The content of trans-anethole, the main component, varied between 81.63% and 87.85%, and the variation was statistically insignificant during maturation stages. Some components, particularly monoterpenes, α-pinene, β-myrcene, limonene, and α-terpinene, varied significantly (p < 0.05) during maturation stages. © 2009 Elsevier B.V. All rights reserved.