Click in the Eyewitness News story window above to watch Eileen Frere's report from the Santa Ana Zoo, including an interview with the zoo director.
The Crested Capuchins are on loan from the Brazilian government where they're listed as a vulnerable species.
With Romeo, Juliet and the baby, there are now 12 of the monkeys at U.S. zoos.
This is the second U.S. zoo birth. So far, Juliet is proving to be a very good mother.
"She spends a lot of time grooming the baby, inspecting them, and it appears to be feeding very well on its own," said Ron Glazier, Santa Ana Zoo director.
The new baby is clinging tight to it's mother, so zookeepers haven't been able to determine the sex.