The essential oil from fresh leaves of Thuja orientalis L. grown in the north-western Himalaya was isolated by means of hydrodistillation and analyzed by GC and GC/MS. Twentytwo compounds representing 94.0% of the total oil were identified. The leaf oil contained α- pinene (29.2%), Δ-3-carene (20.1%), α-cedrol (9.8%), caryophyllene (7.5%), α-humulene (5.6%), limonene (5.4%), α-terpinolene (3.8%) and α-terpinyl acetate (3.5%) as major constituents. The essential oil showed antifungal activity against Alternaria alternata in a direct bioautography assay. Two main bioactive compounds named as b₁ (R_f = 0.54) and b₂ (R_f = 0.80) were observed and tested for antifungal activity; they produced an inhibition zone of 5 and 10 mm in diameter, respectively. The components b₁ and b₂ were further purified by preparative thin layer chromatography and their antifungal efficacy was re-tested. The minimum inhibitory amount (MIA) of b₁ and b₂ against A. alternata was determined as 30.5 and 4.5 μg, respectively, using a bioautography assay. The bioactive constituent corresponding to b₁ was determined as α-cedrol by using GC/MS analysis. The potential of essential oils as a source of natural biocides is discussed