The carriers face weak demand for travel during the recession.
Southwest -- through Monday -- will sell seats to almost everywhere it flies starting at $49 to $99 each way.
Tickets must be bought at least 14 days before flying, travel must be completed by March 11 and seats are limited.
Delta Air Lines appeared to match the Southwest move.
Delta posted a systemwide fare sale on its Web site that runs through Monday -- same as the Southwest sale.
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Southwest launched the sale after this week's announcement that it lost money in the fourth quarter -- its second-straight losing quarter after 69 straight profitable ones.