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Abstract—The goal of the study was to investigate the mechanism of quorum sensing (QS) inhibition in the model bacterium Chromobacterium subtsugae 026 (previously – C. violaceum 026) by Eucalyptus viminalis Labill leaf extract, and to identify the plant-derived components that provide this biological activity. The raw extract showed pronounced anti-QS activity within a broad concentrations range which were an order of magnitude lower than required for bacterial growth suppression (the MIC₅₀/EC₅₀ ratio of 43.6). Chemical analysis of the extract revealed six small plant-derived molecules: pyrogallol, 2,3-dihydro-3,5-dihydroxy-6-methyl-4H-pyran-4-1, p-cymene, 4-((1E)-3-hydroxy-1-propenyl)-2-methoxyphenol, gallic and palmitic acids, whose inhibitory effect on QS in Chromobacterium spp. has previously been shown by in vitro experiments or predicted by in silico models. In addition, minor components γ-caprolactone and γ-octanolactone, structurally similar to acylated homoserine lactones, QS autoinducers in Chromobacterium spp., were found in the extract. Chemically synthesized analogs of these molecules showed anti-QS activity in the C. subtsugae 026 bioassay (ЕС₅₀ = 354.2 and 145.6 µg/mL, respectively). Enrichment of the E. viminalis leaf extract with γ-caprolactone or γ-octanolactone significantly increased the anti-QS activity of these compositions, which was determined by isobolographic analysis as additive or superadditive effects.