She died on Tuesday in her sleep.
Koko, the famous western lowland gorilla who could communicate with humans, has died aged 46.
She died in her sleep at the Gorilla Foundation's reserve in the Santa Cruz Mountains of Northern California.
Researchers taught Koko sign language and she could use 1,000 words whilst understanding 2,000 words of spoken English.
"The Gorilla Foundation is sad to announce the passing of our beloved Koko,"
a Gorilla Foundation press release says. "Her impact has been profound and what she has taught us about the emotional capacity of gorillas and their cognitive abilities will continue to shape the world."
Koko was also fond of cats and kittens and would sit holding them and playing with them.
