U.S. airlines are raising base fares on many domestic flights even though they are getting a windfall from lower fuel prices.
JetBlue Airways raised fares by $4 per round trip on a small number of routes Thursday, and Delta Air Lines picked up and greatly expanded the breadth of the increase, according to FareCompare.com CEO Rick Seaney and J.P. Morgan analyst Jamie Baker.

The hike got an important boost when Southwest matched it a day later. Other airlines often drop price hikes if they are resisted by Southwest, which carries the most domestic passengers.

A JetBlue spokeswoman confirmed that the airline initiated the increase on its domestic flights. Delta said it had matched and expanded "an industry increase." United Airlines and American Airlines, which also owns US Airways, confirmed matching the $4 increase.

Higher base fares don't always result in passengers paying more. Airlines run frequent sales, and they adjust fares on individual flights depending on demand. The last successful broad increase occurred in April.