The enzyme gamma-terpinene synthase has a key position in the biosynthesis of essential oil compounds in the genus Origanum. It catalyses the cyclization of GPP to gamma-terpinene giving rise to the "cymyl" -pathway and the accumulation of gamma-terpinene, rho-cymene, carvacrol, and related compounds in a plant. A comprehensive sequence analysis of a part of the gamma-terpinene synthase gene (intron 2, exon 3, and intron 3) in O. dubium, O. majorana, O. onites, O. syriacum (section Majorana), and O. vulgare (section Origanum) revealed six different variants of the gene that were distinguishable according to sequence characteristics in intron 3. The formation of "cymyl"-compounds was found to be associated with the presence of variants 1 and/or 3 and/or 4 of the gamma-terpinene synthase gene. In plants lacking these gene variants "cymyl" -compounds were completely absent or present in trace amounts only. The alternative "sabinyl" -chemotype (O. majorana) as well as the linalool-chemotype (Turkish O. dubium and O. onites) could originate in a plants' disability to produce gamma-terpinene.