All immigrants who come to the United States have to go through an Alien Registration process according to laws established in 1940. At the end of the process, each immigrant is assigned an Alien Number, which is similar to a Social Security number for people born within the U.S. The Alien Number is sometimes needed when filling out legal documents, and can also be used to help trace ancestry. As long as you have access to the legal immigration documents, you should be able to find this number.
Gather all of your immigration documents, including your green card and any forms you had filled out to legally move to the country.
Look at the green card first. The alien number should be listed near the top of the green card if you came to the country after 1956. You should see “A#” followed by seven to nine digits.
Look through any other immigration papers – such as Form AR-2 – if the green card was misplaced.
Fill out a Form G-1041 Genealogy Index Search Request and submit it to the U.S. Citizenship and Immigration Services office if you cannot find the alien number on any documents. This is perhaps the most efficient way to find the number. The Form G-1041 may be downloaded on the website for the U.S. Citizenship and Immigration Services. As of December 2010, a fee of $20 must be submitted with the form.