Thieves have stolen sheep to the value of £50,000 from a farm near Melton Ross, off the A18, between 7pm on Tuesday, June 2 and 7.15am on Wednesday, June 3.
The offenders forced open a gate to a field on the left side of the A18, heading away from Barnetby Top, making off with approximately 200 sheep and 400 lambs.
Officers are now keen to speak to anyone who was in the area at the time who may be able to help with the investigation.
Pc Tracey Teal said: “It is clear that a large livestock vehicle or livestock type trailer would be required.
“This type of offence causes considerable upset, inconvenience and disruption to the victims, having a high impact on a business which takes years of time and dedication to build up.
“It is very important that the person or people responsible are located and prosecuted.”
Anyone who saw people or vehicles parked in the area at the time is asked to call Humberside Police on 101 quoting crime reference 2117346.
Alternatively, call Crimestoppers anonymously on 0800 555111.