A prison officer at HMP Low Moss in East Dunbartonshire has tested positive for Covid-19 and is isolating at home.
It is understood the officer had recently returned from holiday.
About 40 other staff members at the prison, who were in close contact with the officer, have been tested and are self-isolating but none was positive.
A number of prisoners have also been tested and found not to be carrying the virus.
The prisoners are isolating in their cells for a week.
Visits to prisoners in the affected wing of the jail, near Bishopbriggs, have been suspended but are continuing for other parts of Low Moss.
The prison opened in March 2012 and has a capacity of 784.
It manages male offenders on remand, short term offenders (serving less than four years), long term offenders (serving four years or more), life sentence offenders and extended sentence offenders.