Blue-Green Algae has been found in Lake St. George in Severn Township.
The Simcoe-Muskoka Health Unit says a bloom's been found in the south end of the lake.
Swimming and other water sport activities should be avoided in areas where the bloom is visible.
Exposure could result in illness.
Symptoms from consuming toxins can include headaches, fever, diarrhea, abdominal pain, nausea and vomiting, and can be more serious if water is swallowed in large quantities.