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Quick answer: Fax Form 637 to 855-887-7735 (toll-free), the IRS Excise Operations Unit in Cincinnati, or post it to Mail Stop 5701G, Cincinnati, OH 45999. Send every piece of information your activity letter asks for: the form says a missing item delays the application.
Form 637, Application for Registration (For Certain Excise Tax Activities), is how a business registers with the IRS before it takes part in certain excise tax activities or claims certain excise tax benefits. Most applicants are in fuel: terminal operators, blenders, biodiesel and alcohol fuel producers, and sellers of aviation kerosene. The form also covers manufacturers of some taxable goods, exporters of ozone-depleting chemicals, blood collector organisations, and others.
Each business unit with its own EIN files its own Form 637.
Some common activity letters
You register for one or more activities, each with a letter. This is a selection; the form lists every letter and what it needs.
| Letter | Activity |
|---|---|
| S | Enterers, position holders, refiners, terminal operators, throughputters and certain industrial users of gasoline |
| M | Blenders producing taxable fuel outside the bulk transfer/terminal system |
| X | Pipeline or vessel operators within the bulk transfer/terminal system |
| Y | Buyers of kerosene for use in commercial aviation (other than foreign trade) |
| AF, AB, NB | Producers or importers of alcohol, agri-biodiesel, and biodiesel or renewable diesel |
| AL, AM | Alternative fuelers: selling or using alternative fuel, or producing alternative fuel mixtures |
| UA, UB, UP, UV | Ultimate vendors, such as sellers of aviation kerosene or of fuel for government or nonprofit educational use |
| A | Manufacturers of certain taxable articles, such as taxable tyres, sport fishing gear, bows and arrow parts, or vaccines |
| D | US buyers of certain taxable articles for export |
| BC | Qualified blood collector organisations |
What the IRS checks before registering you
A Form 637 has to be reviewed and approved before you’re registered for anything, and the IRS may require a bond. The review can include an inspection of your business premises during normal business hours, without advance notice.
Each activity letter has its own list of extra information to attach: products, volumes, facility locations, agents and brokers, business partners. Some letters need documents as well, such as an IRS determination letter (letters F and BC) or an FDA blood collector registration (BC). Fuel applicants also answer the ownership-change question on line 8.
A copy of your Form 637 isn’t proof of registration. Only the IRS Letter of Registration is. It lists your registered activities, the effective date and your registration number. Customers and suppliers can check a registration with the IRS’s online Form 637 Registration Status Check.
If you don’t register
The penalty for failing to register when you’re required to is $10,000 for the first failure, plus $1,000 for each day after that you remain unregistered. It doesn’t apply if the failure was due to reasonable cause.
After you’re registered
- Tell the IRS within 10 days of any change to the information you gave on the application.
- A fuel registrant whose ownership changes by more than 50% has to register again.
How to fax Form 637
- Pick every activity letter that applies to the business and list each one in Part II.
- Gather the extra information and documents each letter asks for. Leaving one out slows the application down.
- Have it signed by someone with authority to bind the applicant.
- Add a cover sheet with the business name, “Form 637” and the page count. Our fax cover sheet generator makes one.
- Fax it to 855-887-7735 and keep the transmission report until the Letter of Registration arrives.
EveryFax sends straight from your email, which suits an application with long attachments, and has a two-week trial. Municorn Fax sends from a phone or computer and keeps the confirmation; your first single-page test fax is free.
Do related companies file one Form 637 between them?
What happens if the IRS turns the application down?
How long does registration take?
Faxing another IRS form? See our list of IRS fax numbers, including the forms sent by businesses and tax professionals.
Sources and IRS references
- Form 637 (Rev. December 2025), with its instructions: the activity letters, required information, review, penalty, Letter of Registration and where to send it. Checked 10 October 2026. irs.gov
- 637 Registration Program: registration tests, inspections, the status check and the fax number. IRS page last reviewed 8 September 2026; checked 10 October 2026. irs.gov
