Swine flu has been transmitted to elephant seals living off the coast of California.
The H1N1 strain is the same one that caused a worldwide pandemic in 2009. It is the first time the flu strain has been confirmed in a marine mammal.
"We thought we might find influenza viruses, which have been found before in marine mammals, but we did not expect to find pandemic H1N1," said Dr Tracey Goldstein, who led the US team that made the discovery. "This shows influenza viruses can move among species."
Nasal swabs identified the flu infection in two elephant seals living off the Californian coast. An immune reaction to the virus in another 28 elephant seals indicated more widespread exposure.
The scientists said neither infected seal appeared to be ill, suggesting that marine mammals may carry the virus without showing symptoms.
This raises potential concerns for people who find themselves in contact with the creatures, such as vets and animal rescue workers
How the seals came to be infected remains an unsolved mystery.
"H1N1 was circulating in humans in 2009," said Dr Goldstein, from the University of California at Davis. "The seals on land in early 2010 tested negative before they went to sea, but when they returned from sea in spring 2010, they tested positive. So the question is, where did it come from?"
The seals had been satellite-tagged and tracked, said the scientists, whose findings are reported in the online journal Public Library of Science ONE.
Exposure to the virus probably occurred before the seals reached land, either while at sea or upon entering the near-shore environment.