At the start of World War II, "Ajax" was assigned to the 6th Submarine Division, based in Brest, along with her sister ships "Persée", "Archimède" and "Poncelet". In April 1940, with her sister ship "Archimède", she escorted convoy HX 41 from Halifax to Britain. In front of the advancing German forces, she left Brest along 6:30 pm with thirteen other French submarines and one tanker. The force arrived at Casablanca on 23 June. After the Attack on Mers-el-Kébir, she patrolled along the Moroccan Coast.