The synthesis of a new series of 1beta-methylcarbapenems having pyrrolidine and piperidine moieties is described. Their in vitro antibacterialactivities against both Gram-positive and Gram-negative bacteria were tested and the effect of substituent on the pyrrolidine ring was investigated. A particular compound (IIIb) having hydroxypyrrolidine moiety showed the most potent antibacterialactivity.