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Nonresident aliens, in general, are also liable for Social Security/Medicare Taxes on wages paid to them for services performed by them in the United States, with certain exceptions based on their nonimmigrant status. The following classes of nonimmigrants and nonresident aliens are exempt from U.S. Social Security and Medicare taxes:

F-visas, J-visas, M-visas, Q-visas. Nonresident Alien students, scholars, professors, teachers, trainees, researchers, physicians, au pairs, summer camp workers, and other aliens temporarily present in the United States in F-1,J-1,M-1, or Q-1/Q-2 nonimmigrant status are exempt on wages paid to them for services performed within the United States as long as such services are allowed by USCIS for these nonimmigrant statuses, and such services are performed to carry out the purposes for which such visas were issued to them.
Exempt Employment includes:
On-campus student employment up to 20 hours a week (40 hrs during summer vacations)
Off-campus student employment allowed by USCIS.
Practical Training student employment on or off campus.
Employment as professor, teacher or researcher.
Employment as a physician, au pair, or summer camp worker
Limitations on exemption:
The exemption does not apply to spouses and children in F-2, J-2, M-2, or Q-3 nonimmigrant status.
The exemption does not apply to employment not allowed by USCIS or to employment not closely connected to the purpose for which the visa was issued.
The exemption does not apply to F-1,J-1,M-1, or Q-1/Q-2 nonimmigrants who change to an immigration status which is not exempt or to a special protected status.
The exemption does not apply to F-1,J-1,M-1, or Q-1/Q-2 nonimmigrants who become resident aliens.

For more information, visit: http://www.irs.gov/businesses/small/international/article/0,,id=129427,00.html