Graduate Assistant stipends generally count as taxable income, and the program or unit employing the assistant should require assistants to complete a W-4 tax withholding form, which they should receive from their hiring program or unit.
International Graduate Assistants often find state and federal tax requirements especially difficult to understand. For more information, please consult Human Resources International Staff and Students page Tax Information for Foreign Nationals.
In addition, the Center for International Education maintains a guide to Tax Resources for international students.
See the US taxes and the Glacier Tax Compliance System section of this guide for International Students.