Hassan, H. M., Jiang, Z.-H., Asmussen, C., McDonald, E. and Qin, W. 2014. Antibacterial activity of northern Ontario medicinal plant extracts. Can. J. Plant Sci. 94: 417-424. In the present study, the antibacterial activity (in vitro) of the leaf and/or flower crude extracts of Anaphalis margaritacea (L.) Benth & Hook.f., Grindelia squarrosa (Pursh) Dunal, Apocynum androsaemifolium L., Arctostaphylos uva-ursi (L.) Spreng, Cornus canadensis L. and Xanthium strumarium L. medicinal plants was analyzed through the hole-plate diffusion, minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) assays against Escherichia coli, Aeromonas caviae, Paenibacillus alvei, Micrococcus luteus, Mycobacterium avium subsp. avium and Bacillus cereus bacteria. In addition, the time-kill dynamic processes of these extracts against A. caviae, B. cereus, P. alvei, and M. luteus bacteria were also tested. The leaf and flower extracts of Anap. margaritacea and G. squarrosa possess significant antibacterial activity against all the bacteria tested, with inhibition of A. caviae, P. alvei and M. luteus within 1-12 h of incubation at MBC. Particularly, at higher concentrations of the Anap. margaritacea flower crude extract (2-3×MBC), inhibition of A. caviae, B. cereus, P. alvei, and M. luteus bacteria is achieved between 0.5 and 4 h of incubation. In addition, these extracts exhibit high inhibition diameters (majority > 18 mm) and low MIC and/or MBCs (majority=1.25 mg mL^-1). In contrast, the leaf extracts of Arct. uva-ursi, X. strumarium, Apoc. androsaemifolium and C. canadensis plants demonstrate moderate to low activity. These results indicate that extracts from Anap. margaritacea and G. squarrosa could be a source of novel antimicrobial scaffolds, compounds or pharmacophores with implications in the pharmaceutical industry and medicine.