The public has been urged to avoid the water at Castleblayney's Lough Muckno.
It follows a poor water quality result where an increase in the levels of bacteria was detected in a sample taken on Tuesday of this week.
People are advised not to swim at Lough Muckno, especially if you are pregnant or have a weakened immune system.
Monaghan County Council says these higher levels of bacteria are usually short-lived, and the Local Authority has arranged for further samples to be taken.
Further information can be sought by contacting the Council's Environmental Services.