Butea frondosa has been used traditionally as a topical formulation in the treatment of manydiseases and disorders. Two compounds [BF-1 (crystalline flavonol quercetin) and BF-2 (tannin)from ethyl acetate fraction of ethanolic extract] were isolated from the bark of Butea frondosa. Thestereostructures of the compounds were determined on the basis of chemical and physico-chemical evidence. BF-1 and BF-2 were screened in vitro for possible antibacterial propertyagainst 112 bacteria comprising 3 genera of Gram-positive and 12 genera of Gram-negative types.It was found that both BF-1 and BF-2 exhibited inhibitory activity against several bacteria. Most ofthese strains were inhibited by BF-1 at 50 - 200 µg/ml, while BF-2 (MIC50 400 µg/ml) was muchless active. The bacteria could be arranged in the decreasing order of sensitivity towards BF-1 inthe following manner: S. aureus, Bacillus spp., Salmonella spp., Vibrio spp., Shigella spp., E. coli andPseudomonas spp. The MIC50 of the compound was 50 µg/ml while the MIC90 was 100 µg/ml. Thedecreasing order of sensitivity towards BF-2 was V. cholerae, Bacillus spp., S. aureus, V. parahaemolyticus,Salmonella spp. and Proteus spp. BF-1 was bactericidal in action. In vivo studies with this extractshowed that it could offer statistically significant protection (p < 0.01) to mice challenged with avirulent bacterium. The inhibitory activity of Butea frondosa against Gram-positive and Gram-negativebacteria indicates its usefulness in the treatment of common bacterial infections. The potentiality of BF-1 as an antibacterial agent may be confirmed further by pharmacological studies.