A series of substituted 6,7-dihydro-1-oxo-1H,5H-benzo[i,j]quinolizine-2-carboxylic acids was synthesized and tested for antibacterial activities. Among them, 9-fluoro-6,7-dihydro-5-methyl-8-(4-methyl-1-piperazinyl)-1-oxo-1H,5H- benzo[i,j]quinolizine-2-carboxylic acid (OPC-7241) exhibited potent antibacterial activity against gram-positive and -negative bacteria, including Staphylococcus aureus and