Sheep valued at £30,000 have been stolen from common land in the Welsh Valleys during a spate of thefts.
Rustlers have targeted flocks in the Swansea, Amman and Lliw Valleys, stealing 350 sheep.
The thefts, the first of which were reported last year, have promoted the Farmers' Union of Wales to issue a warning urging farmers to be vigilant.
One of its members, third-generation farmer Howell Davies, has had 62 ewes stolen in the past year.
He blames the rising price of sheepmeat. "Of course, it is not just farm income that suffers from these thefts, as there can also be a loss of valuable breeding lines that are very difficult, if not impossible, to replace," said Mr Davies, of Perthygwynion Farm, Pontardawe.
In 2011, the FUW helped launch Farm Watch, a scheme aimed at improving communication between farmers and the police to reduce farm-related crime.
The FUW's livestock, wool and marts committee chairman, Dafydd Roberts, who farms on Anglesey, urged farmers to get involved with their local Farm Watch scheme and to report any suspicious activity. "This is especially important if such activities are being conducted at night," he said.
Farmers should also make thorough checks to establish the ownership of any sheep purchased, he added. "The union would stress that buyers of sheep or sheepmeat should check the corresponding ownership records and livestock identification to ensure that the animals being purchased are not stolen."
South Wales Police met with the Commoners' Association last November and, as a result, an initiative designed to combat the rise in sheep thefts is set to be launched this spring.
In the meantime, police will carry out spot checks on vehicles carrying livestock and will continue to stop vehicles towing livestock boxes in remote areas late at night.
More on this topic
See our Rural Crime page