Visiting has been curtailed at Letterkenny General Hospital where there has been an increase in diarrhoea and vomiting in two wards.
No visitors are allowed into the affected wards - Medical 3 and Surgical 1 - unless there is an exceptional need.
Hospital management says children should not be brought to the hospital.
It says only essential visits should be made to other wards at the hospital and should be confined to one person at a time.
All appropriate infection control measures are being taken to deal with the situation, according to a statement issued by the HSE, and the visiting restrictions will remain in place until the wards are free from the outbreak.