An A-Number (as it is also called) is a seven to nine-digit number found on multiple types of documents from USCIS. When an A-Number is fewer than nine digits, you simply enter a zero after the "A" and before the first digit, so you have a nine-digit number. U.S.-born citizens and many nonimmigrant visitors may not have alien registration numbers. If you have a green card, the Alien Registration Number will be on the front, usually underneath the USCIS#. For cards issued between 2004 and 2010, the A-Number is listed next to A#. If you do not have a green card, you can find your Alien Registration Number on other immigration documents, such as an Employment Authorization Document, or in the righthand corner of an immigrant visa.