A man has been questioned by police at LaGuardia airport in New York after smuggling a monkey onto a flight from Florida by hiding it under his hat.
Passengers spotted the animal when it climbed out and perched on the man's ponytail, Spirit Airlines spokeswoman Alison Russell told reporters.
Ms Russell said the monkey - a marmoset - spent the remainder of the flight in the man's seat and was well-behaved.
It is unclear whether the unnamed man will face any criminal charges.
The man's journey originated in Lima, Peru. Ms Russell said it was not known how the man avoided detection there, and during a several-hour stopover in Fort Lauderdale, Florida.
When passengers noticed the fist-sized primate on the flight, they asked the man "if he knew he had a monkey on him", Ms Russell said.
New York animal control officials said the monkey appeared to be healthy, the Associated Press news agency reported.
It said the Centers for Disease Control and Prevention was planning to quarantine the animal for a month.
After that, the monkey could be sent to a zoo, AP said.