The Bills, meanwhile, acquired a draft pick in return, and dropped a player who refused to report to Buffalo in a contract dispute. The Bills obtained Walker in March in a trade that sent linebacker Takeo Spikes and backup quarterback Kelly Holcomb to Philadelphia.

Walker, however, refused to report to Buffalo until the team renegotiated the remaining two years on his contract. He was scheduled to make $1.3 million this year and $1.4 million next season.

"When he didn't show up to begin camp, it looked like it would drag on," Bills general manager Marv Levy said. "The opportunity presented itself for us to make this deal with the Bears, so it seemed like the best thing to do. We did it and we're moving on."

Levy added the draft pick the Bills receive will depend on Walker's playing time in Chicago this season. The Bills would've been forced to return Walker to Philadelphia in exchange for a sixth-round pick if the player had not reported by Aug. 5.

Walker had no trouble getting to Bears training camp in Bourbonnais, Ill. Shortly after the trade was announced, the seven-year veteran was taking a physical during the team's afternoon practice.

Walker's agent, Albert Irby, said his client was eager to join the Bears and planned to negotiate a new contract with Chicago.

"Darwin's old deal is gone. The new deal will be talked about tomorrow," Irby said. "We'll take a one-year deal. We'll do whatever it takes. We're not going to hold out."

The Bills offered Walker a one-year contract, but for the same money he was scheduled to make this season. The offer, however, was immediately rejected.

Walker fills an immediate need in Chicago, after the defending NFC champions released Johnson in June because of the player's constant run-ins with the law.

Walker has 26½ sacks over the past five years, making him the fourth-most productive defensive tackle during that span.

The Bears were one of the teams Walker was interested in joining when the Eagles allowed him to seek a trade last spring. But Chicago, at that time, had no room for him because Johnson was still on the team, Irby said.

The Bills hoped Walker would bolster their defensive line, but are pleased with the progress John McCargo has shown in camp. The second of Buffalo's two first-round draft picks last year, McCargo appeared in only five games in 2006 before breaking his foot.

McCargo then had a setback in his recovery and needed a second operation on his foot earlier this year.

"We're very encouraged by the progress John has made. That helps alleviate a little bit of the fact that we weren't able to come to terms (with Walker)," Levy said. "If we could have worked something out with Darwin, we would have liked to have had him."