Antibacterial haloarylmalonamido-1-dethia-1-oxacephalosporins of the following formula: ##STR1## wherein Ar is ##STR2## (in which Hal and Hal' each is halogen and RO is hydroxy or protected hydroxy);COA and COB each is carboxy or protected carboxy including a pharmaceutically acceptable salt group;and Het is 1-lower alkyl-5-tetrazolyl, 1,3,4-thiadiazol-2-yl or 5-lower alkyl-1,3,4-thiadiazol-2-yl,a pharmaceutical or veterinary composition comprising the said haloarylmalonamido-1-dethia-1-oxacephalosporin and pharmaceutical carrier, and a method for treating or preventing human or veterinary infectious diseases comprising administering an effective amount of the said antibacterial compound.