Constituents in rosemary (Rosmarinus officinalis) have been shown to have larvicidal activity against invertebrates. In order to explore the properties of crude extract of rosemary further, we studied the chemical composition and its activity against dichlorodiphenyltrichloroethane (DDT) -susceptible, DDT-resistant, and field strains of Culex quinquefasciatus larvae. The major components of R. officinalis were found to be eucalyptol and camphor, with relative percentages of 10.93% and 5.51%, respectively. Minor constituents included limonene, ( )-4-carene, isoborneol, 1-methyl-4-(1-methylethylidene)-cyclohexene, and pinene. The median lethal concentration (LC₅₀) values of the essential oil of R. officinalis against DDT-susceptible, DDT-resistant, and field strains of larvae of Cx. quinquefasciatus were 30.6, 26.4, and 38.3 mg/liter, respectively. The single median lethal dose (LD₅₀) in Kunming mice was 4752 mg/kg. Essential oils from R. officinalis may, therefore, provide an effective natural plant product for use in mosquito prevention and control.