Abstract The yield and composition variation of the essential oil from Thymus pallescens de NoÃ© were determined. This endemic species growing in Algeria was harvested at three stages of plant development during four successive years (2003â2006). The yield in essential oil showed a maximum in the full flowering stage (4.2â4.6%) and a minimum at the beginning of the vegetative cycle (0.9â1.3%). Analysis was performed by GC and GC/MS. Carvacrol (39.7â65.0%) was found to be the most abundant constituent at each harvest period, followed by p-cymene (5.1â15.1%) and Î³-terpinene (6.1â17.4%). At the vegetative period the essential oil was the richest in carvacrol (51.1â65.0%). At the full flowering period the oils of whole years reached close amounts of the main constituents and of different classes of components. Key Word Index: Thymus pallescens Lamiaceaeessential oil compositioncarvacrolp-cymeneÎ³-terpineneseasonal variation