A flock of 271 sheep, worth up to £25,000, has been stolen from a field in Ramsbottom, Bury.
Thieves herded the sheep through a pen and into the back of what must have been a large vehicle, police said.
The National Famers' Union (NFU) said the scale of the theft was "highly unusual" and that lamb price hikes may make sheep more attractive to thieves.
Farmer William Holden said he felt "like I'd been kicked" when he discovered the theft on 13 May.
The Texel sheep are worth up to £90 each at live auction, said Mr Holden, 50, who inherited Higher Bold Venture Farm at Oswaldtwistle from his mother and now runs it with his two sons.
"Sheep are our livelihood. I just felt gutted, like I'd been kicked, when I drove over to the field at Ramsbottom and realised they'd gone.
"Whoever did it knew all about handling sheep. You couldn't round that number up and drive them all away without knowing what you're doing," said Mr Holden, who has a further 330 sheep and is insured under the NFU.
NFU Northwest spokesman Carl Hudspith said: "Sheep rustling appears to be on the increase, but we're more used to hearing about 10 to 50 being stolen."
Lamb meat prices have risen from £2.70 per kg in 2009 to £4.40 per kg in 2010, said Mr Hudspith.
"The value of the euro means lamb is no longer being imported and so British lamb prices have rocketed, so that may be a reason why thieves want to take sheep," he said.
For live sheep to be sold at auction they must be accompanied by official paperwork and ear-tag identification, which would make it difficult for the thieves to "off-load" their cargo legitimately at auction, he said.
Sgt Ben Hodgkinson, from Lancashire police, said: "It is likely that they knew how to handle the sheep, which would mean they had knowledge of animal husbandry.
"I would ask anyone who saw anything in respect of the theft of the sheep, or anyone who has any other information that might assist in their recovery, to contact the police."