The 51-year-old Zimmer was available because of the Falcons' uncertainty over their next head coach. The Falcons hired Tom Dimitroff as general manager on Sunday, and are still looking for a head coach to replace Bobby Petrino, who quit after 13 games.

Their assistant coaches were free to look for other jobs. The Bengals were looking for help.

Zimmer becomes the third defensive coordinator under Marvin Lewis, who has led the Bengals to the playoffs once during his five seasons as head coach. Poor defense has been a constant thread.

"Mike has coordinated defenses at a high level in the NFL for a number of years, and this is obviously a very important hire for me and for our organization," Lewis said in a statement.

The Bengals planned to introduce Zimmer at a news conference tomorrow.

Defensive coordinator Leslie Frazier was fired after Lewis' second season. Chuck Bresnahan replaced him for the past three seasons, but failed to improve a unit that has regularly ranked in the bottom third of the league.

Zimmer was the Dallas Cowboys' defensive coordinator for seven seasons before moving to the Falcons last year. The Cowboys' defense gave up the fewest yards in the league in 2003.

Atlanta finished 29th in yards allowed and gave up 414 points, fourth-most in the NFL, amid the turmoil of Petrino's departure. Atlanta gave up 34, 37, 30 and 41 points in its last four games.