They were stolen sometime between Friday 11 January and Monday 14 January 2013, when thieves took 30 nine-month-old Texel lambs from a field near Oulston.
The animals are distinctive as they has a large letter “R” painted on their backs.
PC Julia Kessell, of Thirsk Safer Neighbourhood Team, said:
If you were in the Oulston area during the time of the theft I am asking you to come forward and contact the police or Crimestoppers, if you saw any suspicious activity or a vehicle that may have seemed out of place.
I am also keen to speak to anyone who may have been offered the lambs for sale either alive or slaughtered and ask them to get in touch.
I am also asking villagers across the area to keep an eye out for people and vehicles they don’t recognise. If something seems out of the ordinary contact North Yorkshire Police on 101 immediately.
Anyone who can help with this enquiry is asked to contact Thirsk police on 101, select option 2 and ask for Julie Kessell. Alternatively information can be emailed to Julie.firstname.lastname@example.org.
Or call Crimestoppers anonymously on 0800 555 111 quoting reference number 12130009590.