Another ugly loss cost them a chance to move into first place NFC North.
Tavon Austin's 65-yard touchdown run was only the beginning of a big game on the ground for St. Louis, and the Rams beat Chicago 42-21 for their second straight lopsided win.
The Bears, who came in with the league's second-worst run defense, gave up 123 yards rushing in the first quarter and a season-high 261 for the game with defensive stars Lance Briggs and Charles Tillman joining a slew of defensive tackles sidelined by injuries.
"We're never going to say we can't get the problem fixed because it is fixable," defensive end Julius Peppers said. "We're going to simplify it even more than it has been and we're going to keep continuing to work on it until it is fixed."
The Bears (6-5) could have claimed first place in the division with a win on a day when Detroit (6-5) lost to Tampa Bay and Green Bay (5-5-1) managed only a tie with last-place Minnesota.
"We had an opportunity to take advantage of what else happened in our division, but we didn't. That just goes to show you how tough this game is," said Brandon Marshall, who had 117 yards on 10 receptions.
It's gotten to the point that first-year coach Marc Trestman can't trust his defense.
Trailing 24-14, Chicago opened the second half by driving to the 4-yard line. When Josh McCown's scramble was stopped to set up fourth-and-goal at the 1, Trestman saw no other option but to go for it.
Michael Bush was stuffed for a 4-yard loss, with Jo-Lonn Dunbar breaking through the line untouched to blow up the play.
"This was a bit of a track meet out there today and I know that those three points seem like a lot, but I am very confident that it was the right thing to do," Trestman said.
The game featured a brother vs. brother subplot with Rams defensive end Chris Long going up against his little brother, Bears offensive guard Kyle Long. The only time it got really physical, Chris was playing the role of peacemaker.
Racing off the sideline where he'd taken a play off, Chris Long rescued a teammate and dragged his younger brother out of a first-half fracas before it could escalate.
"One way to defuse that situation was to get everybody out of there," Chris Long said. "He happened to be a body I saw, so I grabbed him."
Until they posed for photos after the game with their parents afterward, it had been mostly just another game day. They lined up close to each other all day, and traded the occasional glancing blow.
Kyle Long also blamed himself for missing the key block on the goal line, saying: "I didn't pull far enough. That one falls on me."
On the play that ended with his big brother dragging him off the field, Kyle Long said he'd been reacting to a cheap shot in the back "about 30 yards away from the play." He had defensive end William Hayes down and delivered some forearm blows before Chris Long intervened.
"I still don't know if there was a whistle. Some of the guys said there was," Kyle Long said. "You can't lose your cool, and I lost my cool."
Kyle Long got a personal foul and there were two other major penalties declined on the play, intentional grounding on McCown and a facemask call on fullback Tony Fiammetta.
McCown passed for 352 yards and two touchdowns with an interception for Chicago, which had won four straight in the series. The Bears were dogged by penalties throughout, including a 62-yard punt return for touchdown by Devin Hester nullified by a holding call in the fourth quarter.
Forte had 77 yards on 16 carries and has 6,178 with the Bears, passing Neal Anderson for second on the franchise career list behind Walter Payton, who had 16,726.
Late scores by rookie backup Benny Cunningham and defensive end Robert Quinn, on a 31-yard return of McCown's fumble, provided the finishing touches for the Rams (5-6), who followed up a 30-point rout of Indianapolis in front of their first capacity crowd of the season. Many were clad in Bears orange.
Cunningham rushed for 109 yards on 13 carries, most of it in the second half after Zac Stacy was sidelined with concussionlike symptoms. Cornerback Trumaine Johnson and guard Chris Williams also left following blows to the head.
McCown's passing yardage total was the third-best of his career and most since 2005.
The Rams stunned the Bears early with two big plays and two touchdowns.
Austin reversed field on a pitchout for a 65-yard run that gave them points on their opening possession the first time all year. Stacy's 1-yard run capitalized on a lost fumble by Matt Forte at the 7 for a 14-0 cushion with 12:36 to go in the first quarter.
Jared Cook added a 6-yard catch from Clemens late in the first for a 21-7 cushion, offsetting Martellus Bennett's 7-yard reception from McCown.
"It just seems like we're knocking on the door, the door is a little cracked but we still haven't put our foot in," Marshall said. "It's encouraging. We'll see if we'll be able to get through that door next week. It's really close. It's just a matter of time before we're able to make that jump."