On 31 October 1945, Fall River arrived at Norfolk, out of which she sailed in experimental development operations until 31 January 1946. The cruiser was assigned to JTF 1, organized to conduct Operation Crossroads, atomic weapons tests in the Marshall Islands in the summer of 1946. To prepare for this duty, Fall River sailed to San Pedro, California, where from 16 February to 6 March she was altered to provide flagship accommodations. Arriving at Pearl Harbor on 17 March, she embarked Rear Admiral F. G. Fahrion, commander of the target vessel's group for the tests, and with him sailed in the Marshalls between 21 May and 14 September.