A cephalosporin derivative of the formula (I), ##STR1## wherein A is a mono- or polycyclic heteroaromatic ring, which contains at least one nitrogen atom as a hetero atom and which may be substituted with one or more substituents; R is a phenyl group substituted with at least one substituent selected from the group consisting of an amino group, a hydroxy group, a ureido group and a hydroxymethyl group; and X is an --OCOCH.sub.3 group or an --S-Het group in which Het is a 5- or 6-membered heterocyclic ring containing one to four hetero atoms selected from the group consisting of nitrogen, oxygen and sulfur atoms which may be substituted with one or more substituents, and the non-toxic pharmaceutically acceptable salts thereof which are useful as an antimicrobial and prepared by the reaction of a compound of the formula (II),ho--a--cooh (ii)wherein A is as defined above, or a reactive derivative thereof, with a compound of the formula (III), ##STR2## wherein R and X are as defined above, or a derivative thereof.