The essential oils obtained from the aerial parts and the roots of Conyza canadensis (L.) Cronquist (Asteraceae) at the flowering stage, collected from Turkey were simultaneously analysed by GC-FID and GC-MS. Forty-six components comprising 82.8% of the essential oil from the aerial parts and seven components comprising 96.7% of the roots oil were identified. Major constituents of the essential oil of the aerial parts were found to be limonene (28.1%), spathulenol (16.3%) and beta-pinene (9.7%), whereas cis-lachnophyllum ester (86.5%), (2Z,8Z)-matricaria ester (3.9%) and beta-pinene (2.3%) were identified to be main constituents in the volatiles of the root parts. In addition, antimicrobial activity of the essential oils was screened against Gram-positive and Gram-negative bacteria as well as one strain of fungus by broth microdilution method. Both of the essential oils exhibited antifungal activity against Candida albicans ATCC 10231 with 0.078 mu g/ml MIC values. The highest antibacterial activity was observed by aerial parts oil against Escherichia coli RSKK 234 (MIC: 0.039 mu g/ml). This is the first report on the chemical composition and antimicrobial activity of the essential oils obtained from C. canadensis growing in Turkey.