The synthesis of a new series of 1β‐methylcarbapenems having a 5‐(1,2‐disubstituted ethyl) pyrrolidine moiety is described. Their in vitro antibacterialactivities against both Gram‐positive and Gram‐negative bacteria were tested and the effect of the substituent on the pyrrolidine ring was investigated. A particular compound IIIf having a 1‐methoxyimino‐2‐carbamoylethyl substituted moiety showed the