Carbapenem derivatives useful as antibacterial agents have the formula ##STR1## wherein: X represents a hydrogen atom or a methyl group; and Y represents a group of the formula: ##STR2## in which: Z represents an oxygen atom or two hydrogen atoms; R.sup.1 represents a hydrogen atom, a C.sub.1-4 alkyl group, a C.sub.1-4 alkanoyl group, or a C.sub.1-4 alkanesulfonyl group; R.sup.2 represents a hydrogen atom or a hydroxy group, and R.sup.3 represents a carbamoyl group; or R.sup.2 represents a carbamoyloxy group, and R.sup.3 represents a hydrogen atom or a carbamoyl group; R.sup.4 represents a hydrogen atom or a C.sub.1-4 alkyl group; and ##STR3## represents a 4-6 membered saturated heterocyclic group in which the indicated nitrogen atom is the only hetero-atom; or a pharmaceutically acceptable salt or ester thereof.