aromatic compounds in the steam distilled volatile oil obtained from aerial parts of shrubby basil (hindi - ramtulasi), ocimum gratissimum linn. (lamiaceae) were identified by gas chromatography (gc) and gas chromatography/mass spectrometry (gc-ms). the volatile oil mainly comprises of twenty (20) components, out of which all twenty constituents comprising 100 % of the oil were identified. the volatile oil mainly contains 13 monoterpenes (68.10 %) and bornyl acetate (14.5 %) was found to be predominant constituents followed by -pinene (10.8 %), camphor (9.0 %), c-tagetone (8.6 %), t-tagetone (7.4 %), pinene (4.2 %), myrtenal (4.1 %), -thujene (3.0 %), -camphene (1.8 %), o-methyl eugenol (1.6 %), ctagetone isomer (1.4 %), sabinene (0.9 %) and -terpineol (0.8 %). five sesquiterpene comprising (20.2 %) i.e. -selinene (5.9 %), t-caryophyllene (5.5 %), -guaiene (4.0 %), -gurjunene (2.0 %) and bicyclo germacrene (2.8 %). the two non terpenic components comprise (11.7 %) i.e. ethyl cyclohexenal ketone (7.1 %), n-butyl benzoate (4.6 %). the antimicrobial activity of o. gratissimum oil was studied by cup plate method against both gram-positive and negative bacterial strains and pathogenic fungal species. antimicrobial activity was concentration dependent on shigella flexineri, bacillus cereus and candida albicans.