The present invention relates to new derivatives of 7-(1-pyrrolyl)-1-ethyl-1,4-dihydro-4-oxoquinoline-3-carboxylic acid and 7-(1-pyrrolyl)-1-ethyl-1,4-dihydro-4-oxo-1,8-naphthyridine-3-carboxylic acid of general formula I: ##STR1## in which: X represents a carbon atom or a nitrogen atom, and R represents a hydrogen atom or a fluorine atom, as well as their physiologicaly acceptable alkali metal salts or alkaline earth metal salts. The derivatives of the present invention are advantageously used as antimicrobial agents, espcially as anti-bacterial and anti-fungal agents.