Police have issued an alert to hill farmers after 40 sheep, worth almost £2,000, were stolen from Backbarrow, near Ulverston.
Police say rustling is on the increase in Furness and South Cumbria, where farmers are still trying to restock.
Cumbria Police and the National Farmers Union (NFU) are now warning rural Cumbrian farmers to keep close tabs on sheep and cattle.
Police say the latest theft, of part of a herd of Swaledale sheep, took place at Bigland Hall Estate farm.
It is the second time the farm has been targeted by rustlers this year.
A spokesman for Cumbria Police said: "The theft of sheep is becoming an increasing problem in rural Cumbria.
"We have had a lot of similar reports recently. It is something we are looking at on a force-wide basis as we try to identify who is responsible.
"During the foot-and-mouth period farmers did not have this problem.
"Now they are restocking there is a lot of concern among farmers as the sheep are very valuable.
"Some of the thefts that have been reported are of rare breeds."
Money lost
General manager of the Bigland Hall Estate Tony Myers said: "Earlier this year 40 sheep and 80 lambs were taken from us.
"It's sickening, and obviously we lose out on a lot of money."
A spokesman for the NFU said: "Farmers have had problems over the years as thieves steal their quad bikes and machinery.
"Now there is an increase in the stealing of sheep and cattle, which is very alarming for farmers.
"The loss of stock brings financial implications and it can be difficult for farmers to find replacement stock at this time of year.
"This is yet another nail in the coffin for many farmers coming after foot-and-mouth."