Ethanol extract of flowers of Phrygilanthus acutifolius (Ruiz & Pav.) Eichler (Loranthaceae) inhibited the growth of both Gram (+) bacteria (Staphylococcus aureus, Staphylococcus saprophyticus and Enterococcus faecalis) and Gram (-) bacteria (Serratia marcescens, Acinetobacter sp. and Pseudomonas aeruginosa). This extract was bactericidal against Staphylococcus aureus and bacteriostatic against Pseudomonas aeruginosa. Morphological evidence suggests that the extract causes the swelling of the bacterial body of Staphylococcus aureus, the disintegration of the cell surface and the cell death. Bactericidal activity was optimal at pH 7.5 and was not affected by different ionic strengths. The presence of Mg2+ in the culture medium of Phrygilanthus acutifolius diminished the sensitivity of Pseudomonas aeruginosa strain against the extract. Test results would tend to corroborate the folk belief that the flowers of this plant are efficacious against respiratory infections and would justify its further investigation.