A series of novel 1-alkoxy-1,4-dihydro-4-oxo-3-quinolinecarboxylic acids was synthesized and screened as antimicrobial agents. The most active compounds in vitro against gram-negative microorganisms and Staphylococcus aureus were 1,4-dihydro-1-methoxy-6,7-methylenedioxy-4-oxo-3-quinolinecarboxylic acid (22), 1,2,6,9-tetrahydro-6-methoxy-9-oxofuro[3,2-f]quinoline-8-carboxylic acid (30, and 2,3,6,9-tetrahydro-6-methoxy-3-methyl -2,9-dioxothiazolo [5,4-f]quinoline-8-carboxylic acid (34). These compounds had antigram-negative activity comparable to that of the corresponding N-ethyl derivatives 1, 2, and 4. Their serum levels and urinary recovery rates in rats, however, were significantly improved relative to the latter compounds (1,2, and 4).