The Grand River Conservation Authority says staff have confirmed there is blue-green algae in the waters of the Woolwich and Conestogo reservoirs.
The organization says the algae can release a toxin called microcystin into the water which can make people sick.
Because of the possible presence of the toxin the GRCA is warning residents not to swim in the reservoirs, keep pets and children away from the water, not to eat any fish from the lake and not to use the water for drinking.
So far, there have been no reported cases anyone getting sick from the algae.
