They say if you don't follow the rules, you could face federal charges.
Mountain bikers have been using parts of the base as trails in areas that are off limits.
The southeastern part of Fort Bragg is a federally designated archeological site. Artifacts found over the past decade indicate what life was like for natives from thousands of years ago.
The historical significance is huge, but bikers coming through, digging holes and making obstacle courses are threatening the preservation of the area.
The bikers' lives are also in danger because they are riding in a dangerous military training ground.
"If you can imagine walking up or going down a trail with Concertina wire or razor wire that's been strung up across that trail as a defense position for soldiers and training and hitting that with a bike--bad day," said Bill Edwards, a Fort Bragg Installation Range officer.
Bikers caught trespassing on Fort Bragg face several fines, and destroying archeological artifacts is a federal offense.