AbstractThe antimicrobial activities of the methanol extract of Quercus ilex L. (Pırnal oak) leaves were tested in vitro against a wide range of human and plant‐associated microorganisms. A total of 132 microbial organisms belonging to 55 bacteria and five fungi and yeast species were studied using a disc‐diffusion method and microdilution assays. The results were evaluated as inhibition zones around the disc impregnated with Q. ilex extract at a concentration of 300 µL/mL. The results showed that Q. ilex did not have any antifungal activities against Alterneria alternata, Aspergillus flavus, Fusarium oxysporum, Penicillum spp., whereas there were inhibition effects on the growth of all Candida albicans isolates. In total 97 bacterial strains (74%) were found to be resistant to Q. ilex extract. The remaining 35 (27%) strains of seven different bacteria genera including Brucella, Bacillus, Enterobacter, Neisseria, Pseudomonas and Escherichia were susceptible to the extract tested. The minimum inhibitory concentrations (MIC) of the extract ranged from 125 to 500 µL/mL. These results suggest that Q. ilex possesses compounds with antibacterial and anticandidal properties. Copyright © 2004 John Wiley & Sons, Ltd.