Portstewart Strand has been red flagged by the RNLI after blue green algae was found, making it unsafe for bathing.
The National Trust, who runs the popular North Coast beach, said the presence of the algae meant it was "unsafe for bathing."
"The water here contains blue-green algae, which can cause stomach upsets, skin rashes, eye irritation, fever and muscular pain. It can be fatal to pets, please keep them on the lead at all times," a post by the National Trust said.
"Please protect yourself and your pets by staying away from the algae and keeping out of the water and following the advice of RNLI and the team on the ground," the post said.
The National Trust also said it monitoring the situation with the Department of Agriculture, Environment and Rural Affairs.
Earlier in the week, the Northern Ireland Enivonrment Agency confirmed there had been over 160 confirmed reports of blue green algae since the start of this year.
Many of these have been in the Lough Neagh area - however, DAERA officials have warned that algae blooms are likely to move through the River Bann catchment towards the North Coast.
A report of blue-green algae at a lake in Crossmaglen Co Armagh has also been confirmed by the NI Environment Agency in recent days.
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