Northwest Airlines ( NWACQ) this week became the fifth carrier to charge extra for specific seat assignments and says it was caught off guard by the barrage of newspaper and TV stories.

"We're surprised that some people are saying we're the first carrier to charge for specific seat assignments, when four other carriers already do similar things," said Northwest spokesman Kurt Ebenhoch.

UAL ( UAUA), for example, has said that its strategy to allow coach passengers on United Airlines to upgrade to "Economy Plus" seating could produce about $50 million in 2006 revenue.

For one-time fees ranging from $24 to $99, or for an annual charge of $299, passengers can upgrade to seats with five more inches of legroom. Economy Plus seats may also be purchased or awarded for loyalty flying.

Virgin Atlantic charges $75 for an exit row seat on its trans-Atlantic flights. Las Vegas-based Allegiant Air charges $10 for an advance seat assignment, and Air Canada charges $12 for a seat assignment for low-fare customers.

Starting March 14, Northwest began charging $15 for seat assignments affecting one-third of its exit row seats on its domestic fleet, as well as a handful of aisle seats. The exit row seats have 10 to 13 inches of extra legroom.

Ebenhoch said the program is a test, involving 5% of the airline's capacity that will make desirable seats available at check-in to people who book late. Often, late bookings involve business passengers who pay higher fares, he said. Northwest filed for bankruptcy last September and has been trying to get a handle on its costs.

Among the critics was Gary Kelly, CEO of Southwest Airlines ( LUV) who in an interview on CNBC said: "We don't have a nickel-and-dime approach. We're certainly not charging extra for certain seats on the aircraft."

Kelly said charging extra for good seats is "completely off the table for us."

Ebenhoch said Southwest won't charge extra for select seat assignments because Southwest doesn't have any seat assignments.

"They don't offer seat assignments of any kind," he pointed out, referring to Southwest's open seating policy. "Not for people traveling with infants, not for seniors, not for unaccompanied minors." He said Northwest often matches Southwest fares, and manages to provide seat assignments, as well.

Last weekend, Southwest raised one-way fares by between $2 and $10 in a bid to help offset its fuel expenses. Several domestic carriers matched the increase.