Five monkeys have been stolen during a "targeted" break-in at Blackpool Zoo, police have said.
Two female and one baby cotton-top tamarins and two male emperor tamarins were taken when thieves broke into their enclosures, on Tuesday night.
Lancashire Constabulary's Andy McWilliam said officers were "making inquiries across Europe" as there was "definitely a market for the monkeys".
Zoo director Darren Webster said staff were "extremely saddened" by the theft.
Details of the monkeys have been circulated to all ports and airports in case the thieves try to take them abroad.
"All the animals were born here at Blackpool Zoo and are part of our zoo family, so I would like to urge anyone with any information to contact the police," Mr Webster said.
Cotton-top tamarins are a critically endangered species and a zoo spokesman said both they and the emperor tamarins needed specialist care.
Police said the thieves had cut a hole through a perimeter fence in order to get in.
PC Steve Higgs said that "from the way that these thieves have broken in, this was a planned and pre-meditated crime and the offenders knew what they were looking for".