Several airlines on Tuesday said they joined a fare sale launched by Delta to try to reinvigorate U.S. air travel.
Travel has slowed because of war in Iraq and severe acute respiratory syndrome.
United, Northwest, Continental and US Airways all matched Delta's lowered prices in markets where the airlines compete.
American Airlines said Monday it had matched the fare sale.
The tickets are as low as $44 one-way. They require a week's advance purchase and the lower fares are unavailable for most holiday periods.
The fares are valid for travel Monday through Thursday and on Saturday, from April 14 through Oct. 1. Tickets must be purchased by April 16.