The Sudbury and District Health Unit has confirmed the presence of blue-green algae at the Amphitheatre beach on Ramsey Lake.
Blue-green algae, or cyanobacteria, are microscopic organisms that can increase in number in warm, shallow water.
Some produce toxins that can be hazardous to the health of people and animals.
Signs are posted at the beach advising the public to avoid swimming, drinking water or allowing pets to play in the water while the warning is in effect.
The advisory does not apply to the other beaches on Ramsay Lake.
The health unit says the city's nearby water treatment plant has the technology to provide an effective barrier to all algae.