A unique seven-, eight- or nine-digit number assigned to a noncitizen at the time his or her A-file is created. The 9-digit U.S. Citizenship and Immigration Services number listed on the front of Permanent Resident Cards (Form I-551) issued after May 10, 2010, is the same as the Alien Registration Number. The A-number can also be found on the back of these Permanent Resident Cards.