The Grand River Conservation Authority says blue-green algae has formed in three reservoirs -- Conestogo, Belwood, and Woolwich.
Residents are being urged to be cautious because the algae may contain a toxin that causes illness when ingested by people or animals.
Visitors and cottagers need to avoid swimming in the water and to keep pets and children away from from the algae.
Boating is allowed but don't consume fish from the water, and don't touch the algae.
The warm dry conditions this month may contribute to the development of further blue-green algae blooms.