A third woman hired to look after a gorilla is suing her Californian ex-employer for allegedly ordering her to show her breasts to the animal.
Koko the gorilla can understand 2,000 words of spoken English
Iris Rivera says Gorilla Foundation boss Francine Patterson told her to bare her nipples as a way of bonding with the gorilla, Koko.
Two other women, Nancy Alperin and Kendra Keller, made similar allegations last week in their own legal action.
The Gorilla Foundation, based in San Francisco, strongly denies the claims.
Ms Rivera was an administrative assistant there until she resigned last month.
She is seeking unspecified damages from the foundation for alleged sexual and disability discrimination, invasion of privacy and violation of US labour laws.
Ms Alperin and Ms Keller, who were fired from their jobs, are seeking damages of more than $1m (£528,000) for wrongful dismissal, outstanding overtime pay and sexual discrimination.
All three women allege that Ms Patterson pressured them to expose their breasts to Koko - allegedly on the basis of sign language requests from the gorilla, who is said to use a vocabulary of 1,000 gestures to communicate with humans.
But of the three, only Ms Rivera complied, seeing it as "a disagreeable duty of her employment", said her lawyer Michael Adams, as quoted by the Associated Press news agency.
The women were among 16 employees at the foundation, best known for the 33-year-old female Koko, who has been the subject of several books and television shows.
The foundation's lawyer, Todd Roberts, has said: "We unequivocally deny these allegations and are confident that this case lacks merit."