Isoindoline derivatives represented by the formula (I) and their salts are disclosed. ##STR1## There are many varieties for the compound depending on the types of residues R.sup.1 -R.sup.9 and X. The compounds can be prepared from quinoline derivatives of the formula (II) and an isoindoline derivatives of the formula (III). The compounds of formula (I) and their salts have excellent antibacterial activities against both gram positive and gram negative microorganisms. They can be used as a medicine, an agrichemical, and a food preservative.