A number of 1-ethyl-6-fluoro-1,4-dihydro-4-oxo-7-(4-substitutedcarbamyl-1-piperazinyl) quinoline [or 1,8-naphthyridine]-3-carboxylic acid and their pivaloyloxymethyl esters were prepared. The compounds synthesized were evaluated for antibacterial activity in vitro against Escherichia coli, Bacillus subtilis, Proteus vulgaris, Klebsiella pneumoniae, Staphylococcus aureus and Pseudomonas aeruginosa. Among those 1-ethyl-6-fluoro-1,4-dihydro-4-oxo-7-(4-methylcarbamyl-1-piperazinyl) quinoline-3-carboxylic acid [1] and 1-ethyl-6-fluoro-1,4-dihydro-4-oxo-7-(4-methylcarbamyl-1-piperazinyl)1,8-naphthyridine-3-carboxylic acid [6] showed the most potent in vitro antibacterial activity.