SEPA is advising people not to visit the Loch of Aithness, between Aith and Vementry, after it has tested positive for blue green algae.
Anglers are asked not to fish the loch in the meantime. Warning signs will be posted in the area.
Meanwhile, the Loch of Voe, which had been affected earlier in the year, has been declared free of the algae.
Blue green algae can sometimes cause skin rashes, eye irritations, vomiting, diarrhoea, fever and pains in muscles and joints, while it is known to have killed livestock and dogs.