The Simcoe Muskoka Health Unit is sending out a beach advisory for Gull Lake in Gravenhurst.
This means the water could be unhealthy to swim in due to bacteria in the water, although officials are waiting on test results.
If you choose to swim in the lake during this advisory you should keep from dunking your head underwater or swallowing water.
Bacteria could cause infections on your face or in your nose, ears or throat.