Five rare monkeys taken from a Sussex zoo as they slept could have been stolen to order for illegal collectors.
Jazz and Captain Jack were taken along with baby Larkin
Keepers at Drusillas arrived on Sunday to find the marmosets had been taken.
They were Jazz and Captain Jack, a pair of silvery marmosets, their two-month-old baby Larkin, and two Geoffroys marmosets, Calli and Cartman.
They could be worth hundreds of pounds, but Drusillas officials said the main concern was over Jazz's welfare because she suffers from a medical condition.
Zoo manager Sue Woodgate said: "It requires treatment twice daily.
"She will become very poorly quickly without her medication."
Sussex Police have appealed for information over the break-in.
Two Geoffroys marmosets were among the monkeys stolen
John Hayward, co-ordinator of the National Theft Register, said: "We have had about 50 little monkeys stolen during the last two to three years, no doubt all stolen to order or to feed into the pet trade.
"This year we have had a number of similar thefts, the most recent of which was at Exmoor Zoo where 13 such monkeys were stolen."
Drusillas Zoo Park, in Alfriston, said staff locked up on Saturday leaving the monkeys, a species normally found in the South American rainforest, safe in their enclosure.
But thieves smashed their way in overnight, leaving broken fences and shattered glass.
Deputy head keeper Paula Walling added: "The keepers are very upset about losing some of their animals, especially baby Larkin, the first silvery marmoset baby born at Drusillas."