The National Trus said in a post on social media that potentially toxic blue-green algae had been found at Portstewart and Castlerock beaches.
Department of Agriculture, Environment and Rural Affairs (DAERA) sampling on Wednesday “confirmed” algae at Castlerock and “suspected blue-green algae” at Portstewart strand.
They remain marked as “fully open”.
But the RNLI red-flag status means bathing is not allowed and people should not enter the water under any circumstances.
Blue-green algal blooms have been detected at a number of sites across Northern Ireland during the recent good weather.
It can cause illness in humans and animals, with a number of dog deaths reported recently.
Rising temperatures and pollution create the right environment for an algal bloom to happen.Tags: