Sheep rustlers got away with animals worth around £3,500 from a padlocked field near Garstang.
Police are trying to track down the thieves, who are believed to have swooped on Tuesday, July 9.
The raiders took 25 clipped Texal Cross sheep, valued at between £140 and £180 each, from a flock of more than 70 at Hobbs Lane, Claughton-on-Brock.
A Lancashire Police spokesman said: “The theft was discovered at 2.45pm.
“The owner spent three hours looking around the area, but they have clearly been stolen. There were no holes or gaps in the fencing.
“Whoever did this somehow opened the gates which had been secured with a padlock.
“They will have had a trailer to load the sheep onto before making their escape.
“If anyone has seen someone with a trailer in that area or can help with any information, I would urge them to call 101 and quote the reference LC-20130709-0883.
“These sheep are valued at a minimum of £3,500.”
The raid is the latest in an outbreak of sheep rustling across Lancashire.
Farmers have been urged to join a rural version of the Neighbourhood Watch scheme to combat the rising menace of rustling.
The region is one of the worst in the UK for the theft of livestock and the National Farmers Union (NFU) is advising all rural dwellers to join a local Farm Watch network and share intelligence about such incidents. “If there isn’t one in your area then you could always try and set one up,” said a regional NFU spokesman.
“They really work. We haven’t a clue why rustling is on the increase.
“We don’t know if there is a black market operating in this area for stolen livestock, but the animals are going somewhere.”
The message to join the Farm Watch follows recent advice issued by Garstang Police for horse owners to join the local Horse Watch scheme, to help protect their own and neighbours’ animals from harm.