Abstract: Methanolic, chloroform and aqueous extracts of 26 medicinal plants used in folklore medicine in Saudi Arabia were screened for in vitro activity against some Gram-positive and Gram-negative pathogenic bacteria using the cup-plate agar diffusion method. The extracts at concentration of 0.1 mL cup showed varying degrees of inhibitory activity against the test organisms. Extracts from Withania somnifera showed the highest activity, followed by Datura stramonium, while Zygophyllum portulacoides demonstrated the least activity when compared to 40 μg mL-1 Ampicillin control antibiotic. The bacteria tested differed significantly in their susceptibility to plant extracts, with Staphylococcus aureus and Bacillus subtilis have been completely inhibited. The plants which exhibited a marked antibacterial activity were shown to be rich in flavonoids, tannins and alkaloids. These results support the traditional use of the plants in the treatment of some bacterial infections.