this invention provides novel pyridonecarboxylic acid derivatives having a quite high antimicrobial activity . the derivatives have the following formula : ## str1 ## wherein r 1 , r 2 and r 3 represent each a hydrogen or c 1 - c 6 alkyl group ; r 4 represents an ethyl , 2 - fluoroethyl , vinyl , isopropyl , isopropenyl or cyclopropyl group ; and x represents ch , c - f , c - cl or n wherein one of r 1 , r 2 and r 3 represents a hydrogen or c 1 - c 6 alkyl group , and either r 1 forms a methylene chain having 2 to 4 carbon atoms together with r 2 or r 3 , or r 2 and r 3 form together an alkylene chain having 2 to 5 atoms and r 4 represents an ethyl , 2 - fluorethyl , vinyl , isopropyl , isopropenyl or cyclopropyl group and x represents ch , c - f , or c - cl and salt thereof .