Parts of two beaches in Cornwall have had to be closed.
The jellyfish are about the size of a mussel
Coastguards at Brixham are warning people to stay away from Charlestown Beach, near St Austell, after dozens of small jellyfish were washed ashore.
It is thought many of them are of the "By-the-Wind Sailors" variety, from the same family as the Portuguese Man o' War, which can deliver a nasty sting. They are the size of a mussel.
Meanwhile, Restormel Borough Council has closed part of Porth Beach, near Newquay, after 300 litres of heating oil polluted a nearby river which runs across the beach, near Newquay.
Coastguards are warning people not to handle the jellyfish, which are also known as Valella Vallela. They do give off a minor sting which may cause an allergic reaction.
Annie Westcott from the Brixham coastguard station said: "We're just saying to the public, if you do go down to the beaches, don't handle them but we're quite happy to take reports about them."
Restormel Borough Council says people will be allowed back on much of Porth Beach later in the day.
However, they will be urged to keep away from the river.
Nick Keane from the Environment Agency says everything possible is being done to protect local wildlife in the area and the agency is looking into how the incident occurred.
He said: "We have walked along a majority of the river there's plenty of fish in the river swimming around and they seem to be fairly healthy. There's also a lot of birdlife on the river and it seems to be OK.
Oil polluted a river which runs across Porth Beach
"We have informed the RSPCA and they've been out themselves to have a look."
However, the closure of the beach has angered local businesses. Traders and hoteliers say the area was largely deserted.
One local fisherman, James Goss, says the pollution has caused problems.
He said: "The whole length of one side of the river stank of oil all day. Because the tide was out all day, all across the seaweed and the rocks was stinking."
Officers from Restormel Borough Council and the Environment Agency will be visiting both beaches on Tuesday.