Air One Inc., the small St. Louis-based airline, filed for bankruptcy in Missouri yesterday after a planned sale to Southern Express Corp. fell through.

The company, which had lost more than $40 million since 1983, said the petition for protection under Chapter 11 of the bankruptcy code was in "the best interest of its creditors and other interested parties."

Earlier in the day, Air One had announced it was temporarily halting all scheduled operations because Southern Express Corp., of Austin, Tex., had "failed to meet the financial obligation" of the sales agreement made public Tuesday.

Under that agreement in principle, Air One had said, Southern Express would make "a multimillion-dollar investment and provide additional required capital to maintain Air One Inc. as a viable entity for the forseeable future."

Last week, Air One reported a loss of $13.2 million in the three-month period that ended June 30. Since January 1983, the company has lost $40.2 million.

The company initiated flights in April 1983, offering what it called first-class service at coach prices. It operated flights from St. Louis to New York, Kansas City and Washington and recently announced new flights linking Los Angeles, Chicago and New York, and service between St. Louis and Houston. In a related development, Ozark Air Lines said it would accept tickets from stranded passengers in some of those markets.