The synthesis of a new series of 1beta-methylcarbapenems having methoxyimine and substituted sulfonamide moieties is described. Their in vitro antibacterial activities against both Gram-positive and Gram-negative bacteria were tested and the effect of substituents on the pyrrolidine ring was investigated. A particular compound (IIIc) having methylaminosulfonamide moiety showed the most potent antibacterial activity.