Thieves have stolen 27 Mule sheep from a field above Malham Cove.
Each animal should have an identification tag in its ear with the flock number of UK 125131 and has bright orange marker dye on its shoulders.
The theft occurred between Saturday, October 6, and Sunday, November 4.
Police are urging people to be vigilant, especially leading up to auctions and to report any suspicious activity, vehicles or people, particularly in remote locations or at night.
Anyone with information should contact PC Simon Crossley at Grassington Police Station on 101 quoting crime reference 12120189372.