CONTEXT: Hypericum perforatum L. (Guttiferae) contains many bioactive secondary metabolites including hypericins, hyperforins, and essential oil. OBJECTIVE: The present study was conducted to determine the variation in composition of essential oil in H. perforatum accessions from Turkey. MATERIAL AND METHODS: At full flowering, aerial parts of 30 plants were collected from 10 sites of northern Turkey and assayed for essential oil components by GC-FID and GC-MS. RESULTS: The chemical analysis revealed that the main constituents of the all analyzed samples were hydrocarbon and oxygenated sesquiterpenes such as beta-caryophyllene (4.08-5.93%), gamma-muurolene (5.00-9.56%), beta-selinene (5.08-19.63%), alpha-selinene (4.12-10.42%), d-cadinene (3.02-4.94%), spathulenol (2.34-5.14%), and caryophyllene oxide (6.01-12.18%). Monoterpenes, both hydrocarbon and oxygenated, were represented by scarce amounts of alpha- and beta-pinene, myrcene, linalool, cis- and trans-linalool oxide, and alpha-terpineol. Principal component analysis was also carried out and, according to the results, the first nine principal components were found to represent 100% of the observed variation. DISCUSSION: The chemical variation among the populations is discussed as the possible result of different genetic and environmental factors. CONCLUSIONS: The wild populations examined here are potentially important sources for breeding and improvement of the cultivated varieties.