DETROIT — Union leaders representing the 11,000 pilots at Northwest Airlines and Delta Air Lines continued to meet yesterday to resolve issues concerning the integration of pilot seniority lists.

Developing a single seniority list has emerged as the major obstacle toward a merger of the two airlines, which would create the world's largest carrier.

The Northwest and Delta chapters of the Air Line Pilots Association have told their members that talks are ongoing, but they said they couldn't discuss any details.

The Northwest chapter, whose leaders have been meeting in a hotel near Minneapolis since Thursday, did not return telephone calls yesterday. The Delta chapter, as well as Northwest and Delta, declined to comment on the status of negotiations.

For pilots, few issues are more important than seniority. Pilots' ranks determine almost everything, from pay and work schedules to what planes and routes they fly.

But experts say finding a way to create a common seniority list that both sides can agree on is extremely difficult.

Nevertheless, Northwest and Delta pilots have an enormous incentive to try to reach an agreement.

A Northwest-Delta merger could provide them with multiyear pay increases and an equity stake in the combined carrier that could be worth $729 million, said Vaughn Cordle, chief analyst at the consulting firm Airline Forecasts.

If the pilots prevent a deal from occurring, they will have to wait a few years before they see any increases in pay, he warned.

"The pilot unions would be idiots not to go along with this," he added.

For now, the pilots are in the driver's seat. Northwest and Delta have indicated they don't want to announce a deal until questions about how pilot seniority will be handled have been addressed.

In several previous airline mergers, waiting until after the deal was announced to tackle the issue caused serious problems.

For example, in February 1999, American Airlines pilots staged an 11-day sick-out to express their unhappiness over how the airline integrated pilots from newly acquired Reno Air into their seniority list. The labor disruption cost American $225 million and damaged relations between labor and management at the carrier.

This and other cases of post-merger labor strife show why Northwest and Delta executives don't want to proceed with the deal until their pilots figure out how to work together.

The pilots appear to be taking their time, labor experts said. If the two sides can't reach an agreement, they could bring in a mediator to help resolve their differences.

Reaching an agreement, however, will require sacrifices from both sides, said Gary Chaison , an airline labor expert and professor of management at Clark University in Worcester, Mass.

"Pilots have made a number of concessions over the years, particularly at Northwest," he said. "I don't know if they are willing to give up any more."

He predicts that in the end, the ability to reach an agreement will hinge on whether the pilots stand to gain more than what they may have to give up.