Todd Bradford has taken a job as linebackers coach at the University of Maryland under the new head coach there, Randy Edsall.
Bradford followed head coach Larry Fedora to USM from Oklahoma State at the end of 2007.
Fedora will spend the next week on the road recruiting while he works to find Bradford's successor.
"I want to do it sooner rather than later, but at the same time, we've got to find the right guy to fit with the staff and our philosophy," Fedora said.
Fedora said he hasn't made any decisions yet when he was asked if any current members of his staff will be considered, according to The Clarion Ledger in Jackson.
Defensive line coach Deke Adams, linebackers coach Dave Duggan and defensive backs coach Grady Brown remain on the USM staff.
Fedora said that the preferred type of scheme that a candidate will bring to the job likely won't be a major factor in the search for a new defensive coordinator.
"It's more about the right coach," Fedora said. "We were multiple. We were based out of a 4-3, but we dropped into the 3-4. Just about everybody runs a 4-3 and has a way to slide into another scheme. That part of it is not so much important as it is finding the right coach."
During the 2010 season, which ended with an 8-5 record, the USM defense gave up averages of 29.5 points and 355.7 yards per game.
"I'm obviously going to miss the defensive players at Southern Miss," Bradford said Sunday. "I think we had a very close relationship. We've been through some good and bad times on and off the field. During those kinds of things, I built strong bonds with those guys."