What is another way to say first come served basis?

Here are some alternative ways to say "first come, first served basis":

Formal:

* On a priority basis.

* Based on the order of arrival.

* In the sequence of their receipt.

* According to the time of registration.

Informal:

* Whoever gets there first.

* It's a race to the finish line.

* The early bird gets the worm.

* First in line, first out.

* Grab it while you can.

The best alternative for you will depend on the specific context and the level of formality desired.