Alien Registration Number, or shortly A-Number, is an eight- or nine-digit number generated and assigned by USCIS to noncitizens when they apply for permanent resident status in the USA. You are supposed to indicate your A-Number in all USCIS forms, if you have it. Note that if your A-Number is fewer than nine digits, you should insert a zero after “A” and before the first digit, in order to create a nine-digit number.
You can find your A-Number in the following documents:
– green card,
– immigrant visa stamped in your passport (excluding nonimmigrant visas),
– immigrant data summary and Immigrant Fee handout (you have these papers if you applied for the green card through consular processing),
– employment authorization document (work permit),
– some types of the Notice of Action.
If you still cannot find your A-Number in one of the mentioned documents, you can submit Form G-639, Freedom of Information Act/Privacy Act Request (also called FOIA request) to get a copy of your Alien Registration Number.