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Who must file Form IL-1120-ST and when is its due date?

You must file Form IL-1120-ST, Small Business Corporation Replacement Tax Return, if you are a small business corporation (S corporation) as defined in Internal Revenue Code (IRC) Section 1361(a) that

  • has net income or loss as defined under the Illinois Income Tax Act (IITA) or 
  • is qualified to do business in the state of Illinois and is required to file federal Form 1120S, U.S. Income Tax Return for an S Corporation (regardless of net income or loss).   

In general, Form IL-1120-ST is due on or before the 15th day of the 3rd month following the close of the taxable year. We grant you an automatic seven-month extension of time to file your small business corporate tax return.

See our Subchapter S (Small Business) Corporation web page for more information.                                                                                                                 

If you have additional inquiries, you may submit them on the Questions, Comments, or Requests form.