Syn-and anti-isomers of 7-[2-(2-aminothiazol-4-yl)-2-methoxyiminoacetamido]-cephalosporins were synthesized selectively. These new cephalosporins exhibited excellent activities against a wide variety of bacteria including those which are resistant to β-lactamase susceptible cephalosporins currently available. The syn-isomers showed higher activities than the anti-isomers.