The chemical composition of the essential oilof Thymus zygisL. ssp.sylvestris(Hoffm. & Link) Brot. ex Coutinho obtained from spontaneous plants in the center of Portugal was examined chromatographically using Kovats indices and reference compounds as the methods of component identification. A well-marked chemical polymorphism was detected in these essential oils probably due to genetic factors. Seven pure chemotypes (the linalool, thymol, carvacrol, geraniol/geranyl acetate, 1, 8-cineole/linalool, 1, 8-cineole/thymol and the oc-terpineol/α-terpinyl acetate) and two mixed chemotypes (the linalool-thymol and the cineole-linalool-thyrnol) were characterized.