One of Longleat Safari Park's koalas has been put down after becoming seriously ill.
The female, called Wilpena, was one of five southern koalas to arrive from Australia last October as part of a conservation programme.
The park said it had no alternative after she developed kidney disease.
One male and four female koalas were sent to the Wiltshire park from Adelaide under a scheme to protect the vulnerable species.
They are the only southern koalas in Europe.
Longleat's Graeme Dick said oxalate nephrosis was a disease that affects almost 60% of koalas in captivity and in the wild, but the cause is unknown.
"It wasn't wholly unexpected. We did think at some stage we may encounter the problem, it just hit us quite early on in the programme," he said.
He said Wilpena had undergone two weeks of "really intense palliative care" but her kidneys stopped functioning and "the kindest thing was to let her go".
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Mr Dick said the animals were monitored daily for signs of the disease, but "because they sleep for such long periods of the day it is very difficult to see if they're not very well".
"We're working with the International Koala Centre of Excellence to look at solutions for the problem," he added.
Head of animal operations at Longleat, Darren Beasley, said the keepers were "incredibly upset".