Police marksmen have shot dead one heifer, but a second remains on the loose after the pair escaped from a Darlington cattle market.
The animals escaped from the market early on Thursday
Officers are searching the market town for the remaining animal described as "extremely dangerous".
Commuters and shoppers have been warned to be on their guard as police say the heifer, weighing about half a ton, will attack anyone it sees.
There have been no reports of injuries to the public so far.
A spokesman for Durham Police said: "We cannot stress too highly how dangerous this animal is.
"It will attack anyone it sees and the public must not approach it in any circumstances.
"The animals escaped from Darlington Cattle Market at about 0500 BST on Thursday."
Police say the first animal was shot in the town's Elton Grove.
The remaining heifer was last sighted in the Skerne Park area of the town.