Alien ID
An Alien Registration Number — commonly called an A-Number, and sometimes an Alien ID — is the unique identifier assigned by U.S. Citizenship and Immigration Services to a non-citizen. It is typically seven to nine digits, usually written with an “A” prefix, and it stays with the person permanently across every interaction they have with the immigration system.
It is a file identifier. That is the single most useful thing to understand about it, and it determines what it can and cannot be used for.
| Issued by | U.S. Citizenship and Immigration Services |
| Format | Seven to nine digits, written with an A prefix |
| Assigned to | Lawful permanent residents, asylum seekers, refugees, many visa holders |
| Permanence | Permanent — it does not change with status |
| What it identifies | The person’s immigration file |
| What it does not carry | Current status, work authorization, or validity |
Which documents carry it
The A-Number appears on most documents a non-citizen accumulates, in different positions and under different labels — which is a common source of error in manual processing.
| Document | Where the number appears |
|---|---|
| Permanent Resident Card (Green Card) | Front, labeled USCIS# on current versions |
| Employment Authorization Document | Front, as the USCIS number |
| Immigrant visa and travel documents | On the visa foil or document face |
| USCIS notices and decisions | Top of the notice, as the A-Number |
| Certificate of Naturalization | Retained on the certificate |
Two labeling points cause repeated confusion. On current Permanent Resident Cards and Employment Authorization Documents the field is printed as USCIS# rather than A-Number — the same number under a different name. And the A-Number is distinct from the USCIS receipt number, a case-tracking identifier beginning with three letters that changes with every application. One identifies the person; the other identifies a filing.
What an A-Number proves, and what it does not
This is where the practical risk sits, and the distinction is the same one that applies to any right-to-work credential.
The A-Number identifies a person’s immigration file. It does not encode their current status. The same number belongs to someone whose permanent residence is current, someone whose employment authorization expired last month, and someone whose case was denied. The number is unchanged in all three cases, because it was never a status field.
So an A-Number captured from a document establishes a reliable link to a person and a file. Whether that person is currently authorized to work, to travel or to receive a benefit is a separate question with a separate answer, held by the government rather than by the card.
For employment eligibility the authoritative check is the federal verification system, not the document; for immigration status generally, it is USCIS. A process that captures the A-Number and stops has produced a complete-looking record that does not answer the question the employer or institution is actually required to answer.
Why it matters for identity verification
Three things make the A-Number operationally significant beyond its role as a number on a card.
It is a stable identifier for a population that lacks one. Names transliterate inconsistently, addresses change, and many non-citizens have no Social Security number early in their time in the country. A permanent identifier that persists across documents and status changes is unusually useful for record matching — and it is exactly what a thin-file applicant otherwise lacks.
It reduces false positives in screening. A sanctions screening alert on a shared name is unresolvable on a name alone. A verified document number is the kind of discriminating attribute that closes a name collision outright, which matters disproportionately for populations whose names transliterate several ways.
The documents themselves need authentication. Permanent Resident Cards and Employment Authorization Documents carry substantial security features and are also forged frequently, because they confer work authorization. Reading the number correctly is capture; establishing that the card is genuine and belongs to the person presenting it is verification, and the two are different steps.
What A-Number checks cannot do
They cannot confirm current status. Covered above, and the most consequential limitation.
They cannot be validated by format alone. A correctly formatted A-Number is trivial to fabricate. Only the issuing authority can confirm one is real and belongs to the person presenting it.
They do not apply to everyone. Many non-immigrant visa holders never receive one, so its absence proves nothing about a person’s lawful presence.
They are sensitive data. An A-Number is a government identifier attached to immigration status, which makes it sensitive under several privacy regimes and a category where minimal collection and retention matter.
Frequently asked questions
What is the difference between an A-Number and a USCIS number?
On current Permanent Resident Cards and Employment Authorization Documents they are the same number — USCIS# is simply the field label used on the card. Both are distinct from the USCIS receipt number, which tracks an individual application and changes with each filing.
Does an A-Number expire?
No. It is permanently assigned to the person and stays with them through every status change, including naturalization. The documents carrying it expire; the number itself does not.
Can an A-Number prove someone is authorized to work?
No. It identifies a person’s immigration file and carries no status information. Work authorization has to be confirmed against the authoritative government source — the number and the card alone cannot establish it.
Where is the A-Number on a Green Card?
On the front of current Permanent Resident Cards, labeled USCIS#. Older card versions place and label it differently, which is one reason automated extraction has to recognize the specific card version rather than assuming a fixed layout.
Related reading
- Document capture — how these cards are read and authenticated
- Employment pass — the same status-versus-identity distinction elsewhere
- Machine-readable zone — the structured data on travel and residence documents
- Sanctions screening — where a verified identifier resolves name collisions
