5 C.F.R. § 338.101 Citizenship.
(a) A person may be admitted to competitive examination only if he is a citizen of or owes permanent allegiance to the United States.
(b) A person may be given an appointment in the competitive service only if he or she is a citizen of or owes permanent allegiance to the United States. However, a noncitizen may be given an appointment in rare cases under § 316.601 of this chapter, unless the appointment is prohibited by statute.
(c) Paragraph (b) of this section applies to reinstatement and transfer as well as to other noncompetitive appointments, and to conversion to career or career-conditional employment.