Formulations of compounds repellent to honey bees may be useful tools to keep bees away from areas recently treated with toxic insecticides. Here, we report the activity of a previously unknown honey bee repellent, an essential oil from the flower of Swertia densifolia (Gentianaceae), which has activity toward Indian honey bees Apis florea F. The essential oil was obtained by hydrodistillation and was subjected to a rotating table bioassay with foragers of the honey bee A. florea. Repellency was dose-dependent up to 12 mg/ml and remained constant thereafter. The chemical constitution of the essential oil was examined by gas chromatography-mass spectrometry analyses, which identified 19 constituent compounds. The major constituents (>10% of the oil) were linalool and octadecanoic acid. The constituents of intermediate concentration (5-10%) were a-terpeniol, nerol, n-octadecyl acetate, and spirostan-3-ol. Besides these, five minor constituents (1-5%) and six of the eight trace components present (<1%) have also been identified. The response of A. florea foragers to mixtures of the major and intermediate constituents was studied. Screening of formulations of linalool and a-terpeniol showed repellent properties toward A. florea, behavior which has already been reported by us in earlier studies. The compound n-octadecyl acetate was found to be an attractant, while octadecanoic acid and spirostan-3-ol were neither attractants nor repellents. Nerol, however, elicited dose-dependent effects involving both attraction and repellency. The effects of the constituents of this essential oil were not merely additive, possibly due to synergistic effects. These results establish the previously unknown chemical constitution and dose-dependent repellent nature of the essential oil isolated from flower of S. densifolia toward A. florea and may be used to improve bee management.