People are being urged to keep themselves and their animals out of the Long Tom and Mountain Home reservoirs in Elmore County after blue-green algae was found in the water.
State agencies have issued a health advisory for both reservoirs.
Blue algae can produce dangerous toxins and be harmful to people, pets and livestock.
Those with liver or kidney damage are at an increased risk of illness.
Any fish caught at the reservoirs should be cleaned with fresh water and only the filet portion should be eaten.