The Army sent Fitch to the Manhattan Project's Los Alamos Laboratory in New Mexico. By mid-1944, about a third of the technicians at Los Alamos were from the SED. While there, he met many of the greats of physics including Niels Bohr, James Chadwick, Enrico Fermi, Isidor Isaac Rabi, Bruno Rossi, Emilio Segrè, Edward Teller and Richard C. Tolman, in some cases attending courses on physics taught by them. He worked in the group headed by Ernest Titterton, a member of the British Mission, and became well-acquainted with the techniques of experimental physics. He participated in the drop testing of mock atomic bombs that was conducted at Wendover Army Air Field and the Salton Sea Naval Auxiliary Air Station, and worked at the Trinity site, where he witnessed the Trinity nuclear test on July 16, 1945. He was discharged from the Army in 1946, but continued to work at Los Alamos as a civilian for another year in order to earn some money. He would briefly return to Los Alamos in summer 1948.