450 in-lamb Cheviot ewes have been stolen from a hill farm in the Moffat area of Dumfries and Galloway.
The theft is thought to have happened between 10am on Sunday April 17 and 10:30am on Monday April 18, 2016.
This latest large-scale theft has been valued at about £60,000.
Gemma Thomson, legal and technical policy manager for NFU Scotland, commented: “The frequency and scale of sheep rustling in Scotland seems to be on the increase.
"The logistics involved in a theft of this scale would suggest that this was pre-meditated, which is worrying for the industry.
“We urge farmers to be on their guard and check their flocks more regularly than they normally would.
"We must remain vigilant and be aware of what’s going on around us, reporting anything suspicious to police as soon possible.”
Detective Inspector Scott Young from Dumfries Police Office said: “The theft of such a large amount of sheep would have required careful planning and obviously there must have been vehicles used.
"The grazing land the sheep were on covers several thousand acres north of the A708 Moffat to Selkirk road near to Greymares Tail in Moffat.
“At this time I would urge anyone who saw anybody acting suspiciously in this area between the times stated, or anyone with information that may assist our police investigation to contact Dumfries Police Office through the non-emergency number 101.
"Alternatively CRIMESTOPPERS can be contacted on 0800 555 111, where anonymity can be maintained.”
The sheep are said to have either a red rump or red shoulder with undocked tails.