The Windsor-Essex County Health Unit says swimming is not recommended this week at Holiday Beach in Amherstburg.
WECHU states the weekly test done Wednesday June 4 showed high levels of E.coli bacteria in the waters making it unsafe for swimmers.
Cedar Beach in Kingsville showed elevated levels of E.coli, but not enough to prompt a warning.
The health unit says all beaches remain open.
Beach testing began last week and will run until August at eight local beaches.
Seasonal monitoring includes surveillance of water and weather conditions, as well as weekly water sampling to test for E. coli.