Coach Bo Pelini did not elaborate on the violations committed by the junior from La Porte, Texas. Pelini announced the dismissal following the No. 24 Cornhuskers' practice on Saturday, even though Castille has not practiced with the team for several days.
"It's pretty black and white my expectations and what we lay out as a staff, and if someone doesn't follow those policies and guidelines, they're no longer going to be with the program, and that's the case with Quentin," Pelini said in a statement.
The 6-1, 235-pound Castille was expected to be a key contributor for Nebraska this season, after rushing for 467 yards and six touchdowns last season. He'd been battling Roy Helu Jr. for the starting job at running back.
Castille rotated with Helu and senior Marlon Lucky most of last season, but with both players slowed by injuries, he ran for 125 yards on 18 carries against Clemson in the Gator Bowl. His performance included second-half runs of 58 and 40 yards, and a 20-yard pass reception that helped Nebraska rally for the victory.
The bruising back also ran for 343 yards and four touchdowns as a freshman.
Pelini said he expects freshman Rex Burkhead to become Nebraska's No. 2 running back, but there will be competition for the spot.
"There's a lot of depth and a lot of talent at the running back position. We feel real good about it," Pelini said. "Time will tell. Just like a lot of other positions, there's a lot of competition going on."
Earlier this year, an arrest warrant was issued for Castille after he failed to appear in court on traffic violations. He was later fined for a missing license plate.