Two chimps managed to break their way out of their special enclosure at Whipsnade Zoo.
The zoo had to be closed on Saturday morning as staff tried to round up the chimps, called Jonnie and Coco.
Visitors already in the wildlife park in Bedfordshire had to stay in their cars or were taken to safe places.
Coco was captured unharmed, but keepers could not get hold of 40-year-old Jonnie safely and he had to be put to sleep, said a Whipsnade Zoo spokesman.
Zoo keepers are trying to work out how the pair managed to get out of their enclosure.
The spokesman said the zoo had closed temporarily, but had reopened after staff checked that the enclosure containing the remaining chimps was secure.