Almost every immigration process asks for an A-Number, and almost nobody explains what it is or where to find one. It takes two minutes to learn, and it makes finding a detained relative and starting a federal case dramatically faster.
What it is
An A-Number — short for Alien Registration Number, sometimes written A#, USCIS Number, or Registration Number — is the file number the U.S. government assigns to a person in the immigration system. It begins with the letter A and is followed by nine digits, for example A123456789. Older records occasionally show eight digits, in which case a zero is added after the A.
One person has one A-Number, and it does not change. Every agency uses it: USCIS for applications, the immigration court for proceedings, ICE for detention records. It is the thread that ties an entire immigration history together.
Where to find it
If your relative is already detained you may not be able to ask them directly, so look through whatever paperwork is at home. The A-Number appears on:
The Notice to Appear (Form I-862) — the charging document that starts removal proceedings. The A-Number is printed at the top.
Employment Authorization Document — the work permit card, printed on the front.
Permanent Resident Card — the green card, where it is shown as the USCIS Number.
Any USCIS notice — receipt notices, approval notices, biometrics appointment letters and interview notices all carry it.
Immigration court paperwork — hearing notices and any decision issued by an immigration judge.
Documents ICE handed over at arrest — including the detainer or custody determination form.
If you cannot find it
Do not let a missing A-Number stop you from asking for help. It speeds things up; it is not a precondition.
You can still search ICE's detainee locator using name, country of birth and date of birth. If your relative can call you from the facility, ask them to read the number from their detention paperwork or intake wristband. Attorneys can also request records directly from the government, and a filed habeas petition gives us formal channels to obtain the file.
Why it matters in a habeas case
A federal habeas petition has to identify precisely who is being held and by whom. The A-Number is how the court, ICE and the U.S. Attorney's office all confirm they are discussing the same person — particularly where names are common or have been recorded inconsistently.
It also unlocks the history that decides the case: how long detention has lasted, whether a bond hearing was held, how the person was classified. Those records are indexed by A-Number, and they are what a prolonged detention or denied bond argument is built from.
FAQ
No. A receipt number identifies one application (three letters followed by ten digits, such as MSC0000000000). An immigration court case number identifies one proceeding. The A-Number identifies the person, and stays the same across every application and every case.
Not everyone. It is assigned when someone enters the immigration system — by applying for a benefit, being placed in removal proceedings, or being detained. Anyone in ICE custody will have one, even if the family has never seen it.
It should not. Occasionally a duplicate is created in error and someone ends up with two, which causes real problems because records get split between files. If you see two different numbers on paperwork for the same person, mention it — it can affect a case.
Yes. Information shared with us in seeking legal advice is confidential. The A-Number lets us check the record and give you a straight answer about the options far faster.
Related guide: Find Someone in ICE Detention