Quinolylpropylpiperidine derivatives of general formula (I) in which R.sub.1 is hydrogen or fluorine, R.sub.2 is carboxyl, carboxymethyl or hydroxymethyl, R.sub.3 is alkyl substituted either with phenylthio optionally substituted with halogen, hydroxyl, alkyl, alkyloxy, trifluoromethyl, trifluoromethoxy, carboxyl, alkyloxycarbonyl, cyano or amino, or with cycloalkylthio (3 to 7 members) optionally substituted with halogen or trifluoromethyl, or with heteroarylthio (5 to 6 members and 1 to 4 heteroatoms chosen from N, O and S), optionally substituted with halogen, hydroxyl, alkyl, alkyloxy, trifluoromethyl, trifluoromethoxy, carboxyl, alkyloxycarbonyl, cyano or amino or R.sub.3 is propargyl substituted by phenyl or heteroaryl as defined above and R.sub.4 is alkyl, alkenyl-CH.sub.2-- or alkynyl-CH.sub.2--, cycloalkyl or cycloalkylalkyl, in their various isomeric forms, separate or as mixtures, and also their salts, their preparation process and intermediates and the compositions containing them. These novel derivatives are potent antibacterial agents ##STR00001##