The invention relates to novel 7.alpha.-methoxycephalosporin derivatives having at the 7.beta.-position an amido group acylated by a 1,3-dithietane carboxylic acid residue and to 7.beta.-isothiazolylthioacetamido-7.alpha.-methoxycephalosporins having antibacterial activity by themselves and, as the case may be, capable of being converted into the aforesaid 7.alpha.-methoxycaphalosporin derivatives. The compounds of this invention exhibit excellent antibacterial activity and are particularly effective against gram negative bacteria.