The present study was designated to evaluate the antibacterial activities of aqueous and ethanol:water extracts from leaves, roots and stem bark of Psidium guajava L. The antibacterial activities of the extracts against bacteria were tested by using both microdilution assay. The aqueous extracts of P. guajava leaves, roots and stem bark were active against the gram-positive bacteria Staphylococcus aureus (MICs=500, 125 and 250 µg/ml, respectively) and Bacillus subtilis (MICs=500 µg/ml), and virtually inactive against the gram-negative bacteria Escherichia coli and Pseudomonas aeruginosa (MICs >1000 µg/ml). The ethanol:water extracts showed higher antimicrobial activity as compared to aqueous extracts. Based on this finding, the ethanol:water extract of P. guajava leaves was fractionated on silica gel column chromatography in a bioassay-guided fractionation affording flavonoid mixture, triterpenes (α-and β-amyrin) and sterol (β-sitosterol). Flavonoid mixture showed good activity on S. aureus with MIC of 25 µg/ml. β-sitosterol was inactive for all the bacteria tested.