One beach is closed and swimming is not recommended at three others in Windsor-Essex after water testing showed high levels of E.coli.
Lakeshore Lakeview Park West Beach is closed after water tests showed e.coli counts exceeding 1,000.
Swimming is not recommended at Cedar Island Beach, Colchester Beach, and Sandpoint Beach after tests found E.coli counts above 200.
The Windsor-Essex County Health Unit tests water samples from eight beaches in the region every Wednesday.
High levels of bacteria may cause stomach and intestinal illnesses such as vomiting and diarrhea and may also cause lung, eye, ear, nose or throat infections or skin issues.