To make the election to be treated under Sub Chapter S of the Internal Revenue Service Code a corporation must:
1) Be a U.S. Corporation (one set up under the laws of any state) of another entity that makes an election to be treated as a corporation under the tax code such as an LLC or partnership.
2) All of the shareholders/owners must be U.S. persons for tax purposes. They may not be corporations or non-resident aliens.
3) It must also have 100 or fewer shareholders at all times AND there must be only one class of stock.
Qualifying new corporations have 45 days from incorporation to make a sub-chapter S election. Existing businesses can make the election in the first 45 days of a new tax year.
For more information see Finding the Right Legal Form for Your Business