Surfers were ordered out of the sea at a north Cornwall beach after a suspected shark was spotted in the water offshore.
Lifeguards on jet skis searched the area at Gwithian near St Ives on Wednesday, but found no risk.
Surfers were then allowed to return to the water where there have been sightings of harmless basking sharks in the past.
Beach manager Phil Drew said the evacuation was a precaution.
He said: "This shark had a very large fin so it is going to count out a lot of sharks, like the blue shark.
"Basking sharks have a large fin, so it could have been one of them.
"We have had the odd mako, but they are not regular callers.
"I suspect it was a basking shark because the fin was quit floppy."
Other possibilities included a sunfish, which have been spotted in the area recently.
Mr Drew said: "The surf schools got their students out because they were a bit concerned and quite a few people left the water."