Anyone visiting Andover, Basingstoke and Winchester's hospitals is being urged to be aware of cases of diarrhoea and vomiting.
Hampshire Hospitals NHS Foundation Trust says it's had reports of a number of people falling ill across its sites in recent days.
Anyone suffering from symptoms or who has been in contact with someone with symptoms in the last 48 hours is being asked not to visit.
Staff have been using social media to warn people about the bug affecting its sites.
Anyone who is concerned about what's happening is asked to contact the trust.