Eight apes were found dead on Sunday. Another ape died on Monday after exhibiting "abnormal behavior".
Three of the apes were cotton-headed tamarins. They are small, tree-dwelling apes that are counted among the world's most endangered primates, with fewer than 6,000 animals remaining in the wild.
Among the other dead apes are a Brazza's monkey, a common squirrel monkey, and four saki monkeys.
It is unclear what is behind the sudden ape death. The apes have been autopsied and samples have been taken, according to the authorities. The mammal department has been closed and barriers have been set up.
"While we await the test results, the mammal department will be closed for disinfection and cleaning from today", stated the department for leisure and culture on Monday.
The zoo and the botanical garden form an oasis on approximately six hectares in the midst of the multi-million city of Hong Kong. All other animals in the park are reported to be doing well.
