Antibacterial 7-substituted aminoacetamido oxadethiacephalosporins of the formula: ##STR1## [wherein R is substituted amino, substituted phenyl, or 5- or 6-membered hetero ring; Ar is aryl; Y is hydrogen or methoxy; Het is 5- or 6-membered aromatic hetero ring; and Z is hydroxy or carboxy protecting group] preparable e.g. by acylation of 7.alpha.-methoxy-7.beta.-amino-3-heterocyclic thiomethyl-3-cephem-4-carboxylic acid derivatives, are highly active against gram-positive and gram-negative bacteria, especially those resistant to other cephalosporins and penicillins.