In this paper the bactericidal activities of ofloxacin and its optically active derivative, DR-3355, against non-growing cells of Escherichia coli and Pseudomonas aeruginosa are described. E. coli and P. aeruginosa were killed rapidly by ofloxacin and DR-3355. After treatment with these quinolones, the resting cells of E. coli and P. aeruginosa became plasmolyzed, with apparent cytoplasmic shrinkage without filamentation. Membrane-bound intracellular vacuoles and disruption of the cell envelope were also observed, resulting in extrusion of the cytoplasmic contents. These results indicate that non-growing cells of E. coli and P. aeruginosa were susceptible to ofloxacin and DR-3355, as were logarithmically growing cells.