The Harry Diamond Laboratories has developed a tamper-resistant/tamper-indicating safing system for the U.S. Arms Control and Disarmament Agency. The safing system consists of a fiber optic seal and related equipment that assembles, photographs, and identifies the seals in the field. Such seals are needed for the effective use of containment as a safeguards technique and for the protection of unattended instruments used for surveillance at peaceful nuclear facilities. Described are both glass and plastic fiber optic seals that are reliable and simple to assemble in the field. Existing fiber optic seal inspection equipments are evaluated, and a system is proposed for operational use.