Delta Air Lines increased its 14- and 21-day advance-purchase fares last Thursday and American Airlines followed. By Saturday night, most carriers had matched the increase.
"We didn't match," said Northwest spokesman Jon Austin.
That led US Airways, Trans World Airlines, America West Airlines, United Airlines, and Continental Airlines to rescind the increases Sunday and Monday.
The increases didn't affect sale fares, unrestricted business fares, or international fares.
Northwest has been running an extended fare sale and forcing rivals to match its lower prices while it weathers months of labor unrest and braces for a possible strike by pilots.
"We set our pricing based on our analysis of what the market conditions are," Austin said. "We believe the fare structure that is in place right now is optimal for revenue generation."
PaineWebber Inc. analyst Samuel Buttrick said Tuesday that the labor situation at Northwest, which faces a possible strike by pilots at the end of the month, will be a factor in every decision the company makes over the next three weeks.
In addition to its talks with pilots, which resume Aug. 17, Northwest has open contracts with its five other unions.