The man was taken to a lifeguard station for emergency treatment but was pronounced dead at the scene, according to a statement on the Solana Beach city Web site.
The man's injuries crossed both thighs, San Diego County sheriff's Sgt. Randy Webb said in a statement. Scripps Institution of Oceanography shark expert Richard Rosenblatt says the shark was probably a great white between 12 and 17 feet long.
"It looks like the shark came up, bit him, and swam away," said Solana Beach Deputy Fire Chief Dismas Abelman.
The attack took place about 150 yards offshore. Several swimmers wearing wetsuits were in a group when the shark attacked, said Solana Beach lifeguard Craig Miller. Two swimmers were about 20 yards ahead of the man when they heard him scream for help. They turned around and dragged him back to shore.
Swimmers were ordered out of the water for a 17-mile stretch around the attack site and county authorities sent up helicopters to scan the waters for the shark. The beach was closed for an eight-mile stretch.
"The shark is still in the area. We're sure of that," Mayor Joe Kellejian said.
Hill, a member of the Triathlon Club of San Diego, said he was running on the beach while about nine other members were in the water when the attack took place.
"They saw him come up out of the water, scream 'shark,' flail his arms and go back under," Hill said. "The flesh was just hanging," and Martin may have bled to death before he left the water, Hill said.
Hill said club members had been meeting at the beach for at least six years and never had seen a shark.
Solana Beach is 14 miles northwest of San Diego.
Shark attacks are extremely rare. There were 71 confirmed unprovoked cases worldwide last year, up from 63 in 2006, according to the University of Florida. Only one 2007 attack, in the South Pacific, was fatal.
The last fatal shark attack in California, according to data from the state Department of Fish and Game, took place in 2004, when a man skin diving for abalone was attacked by a great white shark off the coast of Mendocino County.
On Aug. 19, 2003, a woman swimmer was killed by a great white at Avila Beach in San Luis Obispo County on the central California coast.
The last fatal shark attack along San Diego County was off Ocean Beach in April 1994.
