NPS applicants are eligible to enlist if:

(1) Citizens of the United States. Persons who were born in the 50 United States and commonwealths, Guam, Puerto Rico, the U.S. Virgin Islands, the District of Columbia, or who were born abroad on a military Installation or born abroad of U.S. parents who were citizens. All Native Americans and Native Americans who commute between the United States and Canada are U.S. citizens.
(2) Immigrant aliens who have been lawfully and permanently admitted to the United States are eligible for enlistment, however they may not enlist in an MOS requiring a security clearance. (See chap 7, NGR 600-200).
Aliens admitted on a temporary basis are not eligible for enlistment. (i.e. student visa, work only visa, or admitted seeking asylum without permanent status.)

(9) Applicants who possess dual citizenship cannot enlist as both an alien and a citizen. They must choose and declare citizenship of one country and renounce the other. Applicant must have legal permanent resident alien status (thereby renouncing U.S. citizenship) or have filed for proof of U.S. citizenship (thereby renouncing the non-U.S. citizenship).

PS applicants are eligible to enlist if:
(2) Immigrant aliens who have been lawfully and permanently admitted to the United States are eligible for enlistment, however they may not enlist in an MOS requiring a security clearance.  Federal military service includes all combined periods of military service both active and reserve.  Aliens who have been admitted on a temporary basis are not eligible for enlistment. (i.e. student visa, work only visa, or admitted seeking asylum without permanent status.)  NOTE: Per NGB-ARH Policy memorandum 05-042 date 13 June 2005, PS applicant (s) who were previously discharged for failure to obtain citizenship within their original 8 year MSO are eligible to re-enlist provided they meet all other criteria of this ECM.  Army component soldier (s) receiving an RE code for this reason are eligible for re-enlistment.