Around 60 pregnant ewes have been stolen from a farm at Wray, near Hornby.
The sheep, some of which were pedigree Beltex, were taken some time overnight on Friday into Saturday, February 7-8.
Lancashire Police said the animals were marked with a red ‘K’ on their backs and a red mark on the back of their heads.
They also had ear tags with ‘BE’ and ‘UK’ markings because some originally came from Belgium.
This weekend’s theft is the latest in a series being investigated in the Lune Valley in the last six months which have seriously affected farmers’ livelihoods.
Police said the people responsible have access to a livestock transporter and knowledge of handling stock but there was no clear pattern to where and when the crimes had taken place.
Officers believe the offenders carry out “reconnaissance” trips beforehand.
The animals are either taken somewhere to be retagged or sold on or slaughtered for meat.
Anyone with information is asked to contact PC Andy Massingham at Lancashire Police on 101.