Almost 6,500 homes in Westport have been placed on a boil water notice after a suspected outbreak of cryptosporidium.
The precautionary notice was issued by Irish Water to a large number of customers in the town and to those on nearby group water schemes this evening.
It will affect thousands of homes and businesses in the busy tourist town.
Irish Water says the HSE has issued the precautionary boil notice, after eight people in the Westport area reported symptoms of crytosporidium.
They say no crytosporidium had been detected in ongoing water samples, however, as a precaution they are urging customers to boil water before using it for drinking, preparing food and baby food or brushing teeth.
The areas affected are those served by the Westport Water supply as well as the Ballinlough, Cloghan, Brackloon / Bohea, Cushlough, Farnaught and Rosmoney West group water schemes.