A synthesis of 1,4-oxazine and pyrazine ring systems by an intramolecularcyclization of 7-substituted 1-ethyl-6-fluoro-1,4-dihydro-4-oxo-1,8-naphthyridine and -quinolinederivatives having a nitrogen or oxygen nucleophilic site in the C-7 appendage was studied. The in vitro antibacterial activities of compounds prepared by this method were tested.