The synthesis and biological activities of optically active new 2-isocephems with a 1,3-dihydroxy-4-pyridone moiety at C-7 are described. They were found to have potent antibacterial activity against gram-negative bacteria including Pseudomonas aeruginosa. Among them, 3-[(4-methyl-5-carboxymethyl)thiazol-2-yl]thiomethyl derivative 10d possessed excellent anti-pseudomonal in vitro potency and in vivo efficacy.