New York ? Three of the nation’s largest airlines raised the price of round-trip fares for leisure travelers less than a week after the Memorial Day holiday, the official beginning of the summer travel season.

Continental Airlines initiated the $20 fare increase late Thursday, and Delta Air Lines and American Airliners matched it Friday. Frontier Airlines, a small budget carrier, also matched the increase.

The change, which applies to advance-purchase tickets requiring a Saturday night stay, resembles a fare hike announced by several carriers in April. That increase was dropped when others in the industry didn’t follow suit.

“With the lack of high-paying business travelers, the burden to make an airline profitable now falls on the group that is flying the discretionary leisure travelers,” said Tom Parsons, chief executive of Bestfares.com.

The increase is the third time since mid-April that Continental has attempted to raise advance-purchase fares by $20.

“All of the attempts have failed because Northwest Airlines opted not to match the fare hikes by the other major airlines,” Parsons said.

Air travel has increased incrementally since the sharp dropoff after Sept. 11, however passenger demand was still down 12 percent in April compared with a year earlier.

Carriers have tried to spur demand by keeping prices low. The average price of domestic airfare was down roughly 12 percent in April, compared with a year earlier, with a 1,000-mile domestic trip costing $124.80.