The synthesis of a new series of 1beta-methylcarbapenems having cyclic sulfonamide moieties is described. Their in vitro antibacterial activities against both Gram-positive and Gram-negative bacteria were tested and the effect of substituent on the pyrrolidine ring was investigated. A particular compound (IIIi) having 2-methyl-[1,2,6]thiadiazinan-1,1-dioxide moiety showed the most potent antibacterial activity.