What is the 9-digit A-Number on Form N-400?
An A-Number (Alien Registration Number) is the identifier USCIS assigns to many immigration records. Form N-400 asks for it so USCIS can match your naturalization application to your file. It is often printed as 8 or 9 digits, sometimes with a leading “A”.
Based on official USCIS sources
CivicNest editorial. CivicNest is not a law firm and is not affiliated with USCIS.
- Form
- N-400
- Edition
- 01/20/25
- Last reviewed
- September 10, 2026
What an A-Number is
USCIS uses “A-Number” to mean Alien Registration Number. It is a file number on many immigration documents, including the Permanent Resident Card (Green Card) for lawful permanent residents.
Common formatting
People often describe it as a 7- to 9-digit number. USCIS documents may print it with or without a leading “A” and may omit leading zeros in some displays. Enter the number the way the Form N-400 instructions and the form fields require. CivicNest will ask for the same identifier during preparation.
This page does not show sample A-Numbers. Do not copy an example from an unofficial website.
Where you may see it
Look on your Green Card, USCIS account notices, and other USCIS correspondence. The exact label can read “USCIS #”, “A-Number”, or “Alien Registration Number,” depending on the document.
If a form asks for one
If Form N-400 asks for an A-Number, use the number from your USCIS documents. If you cannot find it, follow the Form N-400 instructions rather than guessing. Do not invent a number.