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By EDWARD WONG

DENVER — From the control tower of Denver International Airport, Tim Thornton stared down more than 300 feet at the white gates that snaked like tentacles from Concourse A.

Mr. Thornton, director of Frontier Airlines' operations here, pointed to his company's 10 gates to the left of the tower. In a half-hour, at 4 p.m., the airline would use six of those to funnel passengers to flights.

Then Mr. Thornton waved a hand to his right, at the eight gates used by United Airlines to load regional jets. In a half-hour, only two planes would be parked there. That was the way it always was, Mr. Thornton said: Frontier's planes constantly taxiing in and out of the terminal, and United's presence barely noticeable.

"They're not using the gates to their fullest," Mr. Thornton said. "We could do better."

Mr. Thornton's insistence that United give up its gates is one part of a three-way struggle among United, which is reorganizing its business in bankruptcy court; Frontier, a growing low-cost airline that is United's main rival here; and the city of Denver, which owns and operates this sprawling airport that was completed in 1995 at a cost of $5 billion.</font>