Northwest and Continental already have a code-sharing agreement for their flights, noted Bill Harrell of Harrell Associates, an airline industry consultant.

As a result of the collective increases, the average one-way coach fare was $98 this week, according to Harrell Associates, an industry consulting firm that tracks air fares.

Robert Harrell, the president of Harrell Associates, said the latest increases reflect efforts by the major airlines to adjust fares after the swift cuts they made in Delta's wake.

"Anything that is discouraging to travel, all other things being equal, will put downward pressure on fares," said Bob Harrell, an industry consultant with Harrell Associates in New York.

"It's an amazing story when you look at where US Airways was over the last three to four years," said the airline consultant Bob Harrell of Harrell Associates in New York.

At the same time, advance-purchase fares are already down 20 percent this year from last, according to Harrell Associates, a New York consulting firm, with the best deals on the increasingly competitive transcontinental routes.

"It's almost like the dilemma of refinancing a mortgage," said Bob Harrell, a principal with Harrell Associates.

This latest increase will affect some of the most expensive flights, which have already increased 21 percent in the last year, according to Harrell Associates, an airline data-tracking firm.

Bob Harrell of Harrell Associates in New York, an airline and travel consultancy, said a consumer backlash against an overload of special offers was possible.

The average one-way business fare has fallen 36 percent over the last three years, to $431 last month from $676 in September 2002, according to Harrell Associates, a New York company that analyzes air fares.