A temporary Do Not Swim notice has been put in place at Bertra beach near Westport.
People are being urged not to swim at the beach, following the detection of E.Coli in a water sample taken yesterday.
Mayo County Council says the bathing water at Bertra will be re-sampled, with results expected by Friday.
Head of Environment with Mayo County Council Martin Keating told Midwest News that the test result is disappointing, following excellent water quality results from the previous number of samples.
He said investigations are ongoing to identify the possible cause of the contamination.
Mr Keating points out that there are a number of breaches with excellent water quality around Clew Bay, which will provide alternatives for swimmers for now.