Abstract: Compounds represented by the following formula (I), that is, cephem derivatives having substituted or unsubstituted imidazo[5,1-b]thiazolium-6-yl as a substituent at the 3-position of the cephem ring: ##STR1## wherein X represents CH or N, R.sup.1 represents H, alkyl, alkenyl or the like, and R.sup.2, R.sup.3, R.sup.4 and R.sup.5 represent H, alkyl, carbamoyl, amino or the like. The above compounds have excellent antibacterial activity, and useful as medicines for the treatment of various infectious diseases caused by bacteria.

Abstract: A cephalosporin derivative having a quaternary salt type substituent group at the 3-position, represented by formula (I): ##STR1## wherein X is a carbon atom or a nitrogen atom; Y is a sulfur atom, an oxygen atom or a nitrogen atom substituted with a substituted or unsubstituted lower alkyl group; R.sup.1 is a hydrogen atom, a lower alkyl group or a substituted lower alkyl group; R.sup.2 is a lower alkyl group, a substituted lower alkyl group, a lower alkylene group or a substituted lower alkylene group; and A is an unsaturated six-membered heterocyclic ring containing at least one nitrogen atom, or a pharmaceutically acceptable salt thereof is disclosed. The derivatives have excellent antibacterial activities and can be used as a drug for the treatment of various bacterial infections.