More than 40 Cheviot ewes were stolen from common land near Hay on Wye, Dyfed-Powys Police said.
More than 40 sheep have been stolen from Hay-on-Wye, police have said.
The Cheviot ewes were taken from the Black Mountains common hill.
Officers from Dyfed-Powys Police's Rural Crime Team said the theft is believed to have taken place between August and October.
Anyone with any relevant information that can help this investigation, are urged to contact the police by telephoning 101 or alternatively Crimestoppers anonymously on 0800 555 111.
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