Antibacterial 7.beta.-arylmalonamido-7.alpha.-methoxy-3-heterocyclic thiomethyl-3-cephem-4-carboxylic acids of the formula: ##STR1## wherein Ar is a heterocyclic group or p-oxysubstituted phenyl; COB.sup.1 and COB.sup.2 are each a carboxy or protected carboxy group; and Het is an unsubstituted or substituted heterocyclic group. These compounds are prepared from, e.g., 7.beta.-amino-7.alpha.-methoxy-3-heterocyclic thiomethyl-3-cephem-4-carboxylic acid derivatives by acylation with arylmalonic acid or reactive derivatives thereof. They are useful as an active ingredient in antibacterial pharmaceutical preparations.