Hinokitiol, the major component of Aomori Hiba (Thujopsis dolabrata Sieb. et Zucc. var. hondai M_AKINO) has already been found to show strong antibacterial activity. In this study, its mechanism of action was examined using EscherIchia coli IFO 3301 and Staphylococcus aureus IFO 12732. As a result, it was suggested that at least a part of the mechanism of the bactericidal action of hinokitiol was due to the metabolic inhibition of the cell membrane in connection with the permeability of the cell membrane and respiration from the following three points. Firstly, from examination of the portraits of electron micrograph of E. coli IFO 3301 and S. aureus IFO 12732, no morphological change was found in the group treated hinokitiol as compared with the control group. Secondly, the respiration of both bacteria was remarkably suppressed by the addition of hinokitiol. Thirdly, incorporation of [U-¹⁴C] -labeled adenine into low molecular and nucleic acid moiety of cell bodies of both bacteria was strongly inhibited by the addition of hinokitiol. Strong suppression was also found in the incorporation of [U-¹⁴C] -labeled amino acid mixtures into low molecular and protein moieties of cell bodies of both bacteria treated with hinokitiol.