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The chemical characterization and antimicrobial activities of Dittrichia viscosa (L.) essential oils extracted by hydrodistillation technique from leaves and stems are reported in this work. Dittrichia viscosa (L.) samples were collected from the Bainem forest in the northwest part of Algiers (the capital). Gas chromatography-mass spectrometry (GC-MS) analytical method was employed to identify the oil's chemical composition. It was found that leaves are mainly composed of three major abundant composites, specifically, caryophyllene oxide (10.4%), fokienol (9.6%) and trans-nerolidol (7%). Moreover, the oil isolated from the stems was found to be chiefly composed of trans-totarol (18.1%), a-cedrol (16.7%), and ferruginol (16.6%). Additionally, antimicrobial activity tests were performed on the isolated essential oils using the zone of inhibition (agar disk-diffusion method) to determine the minimum inhibitory concentration (MIC) of four bacteria strains, mainly, Bacillus subtilis, Staphylococcus aureus, Pseudomonas aeruginosa, and Escherichia coli. The MIC values of leaves are found to range between 15 µg and 100 µg, whereas stems are found to exhibit MIC values ranging between 20 µg and 300 µg. Furthermore, antifungal susceptibility tests, which become important tools to dictate the treatment of fungal diseases, are conducted on two yeast strains: Saccharomyces cerevisiae and Candida albicans. The obtained antimicrobial results are correlated with the chemical composition findings of the essential oils from leaves and stem to determine the roles of the chemical composites on the antibacterial activity. Interestingly, the oil obtained from the leaves displayed a better inhibitory effect on (bacteria and yeast strains) in comparison with oil (stems). This difference in inhibitory effect can be attributed to the dominant existence of the oxygenated sesquiterpenes and trans-nerolidol compounds in leaves. [ABSTRACT FROM AUTHOR]