Sheep rustlers have stolen more than 400 animals from farms on the North York Moors in the last month.
Sheep thieves are returning after the foot and mouth crisis
In the biggest raid, 348 sheep were taken from a farm at Aislaby, near Whitby in the early hours of Wednesday morning. Of those, 251 were in lamb.
Another 71 disappeared in Danby and 20 ewes in lamb were stolen in Helmsley.
North Yorkshire Police said sheep rustling, which virtually ceased after the foot and mouth crisis of 2001, was becoming a problem again.
"After foot and mouth, the sheep thieves vanished," said a police spokesman.
"But within the last year it has come back and has been getting worse again."
Last autumn, farms in Wensleydale and Swaledale were targeted by thieves, with the rustling put down to rising livestock prices.