About 80 sheep have been stolen in Knighton in Powys, say police.
The Dyfed-Powys force said the animals were taken from the Beacon Hill area while they were grazing on common land between 31 July and last Friday.
Officers warned locals to be vigilant and said the majority of people who committed offences in rural areas were travelling criminals.
In August, farming leaders revealed that rustling incidents in the UK had almost doubled in six months.
Pc Bill Cowan of Knighton police said: "Unfortunately, the vast majority of rural crime offenders are not local and they often travel some distance to commit their crimes in these isolated areas.
"Pay attention to people who you're unfamiliar with or suspicious of. Make a note of their appearance and the details of any vehicles that they are using."
In August, farming leaders urged farmers to be vigilant after a rise in sheep rustling incidents.
The Farmers' Union of Wales said offences had "escalated significantly in recent years".
It joined forces with the National Farmers' Union (NFU) Cymru and others to launch Farm Watch to cut crime.
According to NFU Mutual, the union's insurance wing, 32,926 sheep had been stolen from farms across Britain since January.
The surge in sheep thefts has been blamed on the high price of lamb, which has seen prices double in three years. Prize breeds can sell at auction for thousands of pounds.
A total of 38,095 sheep were stolen in Britain throughout 2010, but that could be higher, as a third of farmers are not represented by the company.
People with information about the stolen sheep in Knighton are asked to contact Pc 914 Trish Dixon on 101 or 01267 222020.