Tropolone was shown to be bacteriostatic and bactericidal for a wide range of bacterial species. This antibacterial activity was quantitated using standard methods. Tropolone treatment of whole cells resulted in cell lysis characterized by bleb formation and subsequent loss of cell contents after rupture of the bleb. Although bleb formation and lysis did not occur in the presence of 20% sucrose, cell viability was lost. Spheroplasts and protoplasts also lysed in the presence of tropolone. These results indicate that tropolone acts on the cell wall or envelope and on the plasma membrane.