If you want to stretch out on Continental Airlines, it may cost you.

Continental has become the latest airline to start charging coach passengers for seats with extra legroom.

Starting March 17, passengers in coach can buy a seat in the exit row and grab at least 7 more inches of space. Passengers who belong to Continental's frequent-flier program and fly at least 25,000 miles a year, and those traveling with them, will continue being able to select those seats at no additional cost.

"Our customers want more choices," Jim Compton, Continental's executive vice president and chief marketing officer, said in a statement. "Seats with additional legroom are higher-value seats, and we want to offer them to customers who recognize that value."

Passengers can buy the seats when they check in online or at the airport up to 24 hours before their flights. The costs will vary. Such a seat on a flight from Newark Liberty to Bush Intercontinental Airport in Houston might cost an extra $59, for instance, while additional legroom might be available on a holiday at a discount, the airline says.

Continental isn't alone in putting a premium on the most desirable seats as a way of bringing in extra revenue. JetBlue and AirTran are among the airlines that charge more for extra legroom or a seat in the exit row, according to www.SmarterTravel.com, which tracks fees in the industry.

US Airways, with its "Choice Seats" program, allows passengers to buy an aisle or window seat for premiums that range from $5 to $30.

And on Virgin America, main cabin seats that offer additional legroom in some exit rows and the bulkhead are a separate class of service called "Main Cabin Select."

Passengers who pay for the upgrade get nearly 6 inches of extra space as well as complimentary food and drinks.