A great white shark broke free from a fisherman’s line and then swam off and savaged a long-distance swimmer.
The unnamed victim was taken to hospital with wounds to his stomach and chest in the attack off southern California’s Manhattan Beach.
One report said he is in a critical condition.
The incident began when a man fishing off Manhattan Beach Pier hooked the 10ft (3m) shark.
He spent 30 minutes trying to reel in the shark before the line broke.
Los Angeles County Fire Department said the same agitated shark attacked the victim about 40 minutes later.
He was training with a group of long-distance swimmers about 300 yards (270m) from the pier when he was bitten.
A surfer put him on his board and took him ashore.
Another surfer, Aram Ozen, said at first people thought the victim was having trouble swimming.
Then they heard the swimmers screaming “white, white!” referring to a great white shark.
“It was a scary scream,” Mr Ozen told the LA Times. “It was kind of freaky. There were a lot people screaming back to shore.”
The attack follows a series of shark sightings in the area.