A three-metre-long shark forced 21 swimmers to abandon their ocean race in Australia after its fin was seen following them through the water.
Boats and jet skis were used to rescue the swimmers taking part in the 10 km (six-mile) Cole Classic race in Sydney.
A team of other boats were used to chase the shark away from the shore at Manley Beach.
The rescue came three weeks after three sharks killed a 21-year-old Australian woman off Brisbane.
Some of Australia's top long-distance swimmers were involved in the race.
Race director Stephen Ford said: "The shark was on the surface so the best thing we could do was start a mass evacuation straight away."
The woman who died off the coast of Brisbane was the 10th to be killed by a shark in Australian waters since 2000.