An alien, 18 years or old, visiting the United States is required by US law to carry, at all times, any “alien registration document” issued to that alien by US immigration officials. 8 U.S. Code § 1304 - Forms for registration and fingerprinting (subsection e). For permanent residents, this is the I-551 “green card”, and a green card holder is required to possess this document at all times.The situation is much murkier for nonimmigrant aliens. The document that most nonimmigrant aliens are “required” to carry is Form I-94, the “record of entry” that is created when the alien enters the United States. The problem with this requirement is that, under current practice, CBP border agents generally do not issue a paper I-94 to a visiting alien; only an electronic document is created. If you have entered the United States as a nonimmigrant alien and were not given a paper I-94, then you are not required to carry any document because none was issued to you. If, however, you were issued a paper I-94 or any other comparable “alien registration” document when you entered the United States, you are required by federal law to retain and carry that document in your possession at all times while you are in the United States.You are never required to have in your possession your passport or the visa, if any, you last used to enter the United States, except that you should have your passport in your possession when you leave the United States. If you were issued a paper I-94 when you entered the United States, you are expected to surrender it upon leaving.However, you should be aware that if you do not have your passport with you during your travels within the country, you may have difficulty proving that you are an alien national entitled to the privileges and immunities that alien nationals are entitled to under international law (such as the right to consult with consular officials upon being arrested). You may also find it difficult to prove your identity (which is required for domestic air travel and may be required for other purposes as well). You may find it desirable to secure your original passport and other travel documents in a safe place, and carry a copy of the photograph page, visa page, and (if you received one) I-94 admission record with you, especially if you are going to be in the United States for an extended period of time. At the very least, have some documentation of your alien registration number (A-number) and your I-94 document number, since those can be used to verify your right to be present in the country, should it be necessary to do so.