The latest testing on water samples from seven public beaches in the region has prompted the Windsor Essex County Health Unit to shut down Colchester Beach.
The e.coli count was 1000 ppb after samples were collected earlier this week.
Swimming is not recommended at Point Pelee North West Beach and Holiday Beach after they posted results of 419 ppb and 226 ppb.
The health unit posts warnings at beaches whenever the e.coli count is over 200 ppb.
Swimming is safe at the other four beaches in the region, Cedar Beach, Cedar Island Beach, Seacliff Beach and Lakeshore Lakeview Park West Beach.