Acanthamoeba castellanii is a ubiquitous organism found in environmental water. The amoeba is pathogenic to toward humans and is also a reservoir of bacteria of the genus Legionella, a causative agent of legionellosis. Oakmoss, a source of natural fragrance ingredients, and its components are antibacterial agents that are specifically active against the genus Legionella. In the present study, oakmoss and its components were investigated for their inhibitory effects on total (extraand intracellular) Legionella pneumophila within A. castellanii and on L. pneumophila within A. castellanii. Among the oakmoss components, 3-hydroxy-5-methylphenyl 2,4-dihydroxy-6-methylbenzoate (1) , 3-methoxy-5-methylphenyl 2,4-dihydroxy-6-methylbenzoate (2) , and 8-(2,4-dihydroxy-6-(2-oxoheptyl) phenoxy) -6-hydroxy-3-pentyl-1H-isochromen-1-one (8) reduced the number of total bacteria (extraand intracellular) in a test culture and also exhibited high amoebicidal activity against L. pneumophila within A. castellanii at concentrations lower than their IC 50 values for A. castellanii. In contrast, 6,8-dihydroxy-3-pentyl-1H-isochromen-1-one (5) reduced the total number of L. pneumophila and, also that of total bacteria after 24 h of treatment (P < 0.05) , whereas the compound did not exhibit amoebicidal activity against L. pneumophila within A. castellanii at concentrations lower than its IC 50 value against A. castellanii. Thus, it is suggested that these oakmoss components could be good candidates for disinfectants to protect from Legionella infection.