The U.S. Department of Transportation fined Southwest Airlines (NYSE:LUV) $200,000 by on Tuesday for advertising low airfares on flights that in one case the airline didn't make enough low-fare seats available, and in other cases didn't make any cheap seats available at all.

The DOT found the airline had advertised one-way nonstop fares "for $100 or less" for travel this past Feb.14 but hadn't made a reasonable number of those seats available.

And on Jan. 30, Southwest advertised $66 one-way fares from Dallas to Branson, Mo., for flights between March 1 and March 21, but no sale fare seats were available on any day during that period.

DOT found that Southwest violated the agency's full-fare advertising rules, which were established to prevent and punish deceptive airfare advertisements. DOT has ordered the carrier to cease and desist from engaging in further prohibited unfair and deceptive practices.

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