On Monday evening, 15 homes were evacuated and four people were taken to a rest centre, while others went to stay with family and friends.
A village hall is now being used to house residents who are unable to find alternative accommodation.
North Wales Fire and Rescue Service said about 30 homes in Bethesda, Bangor, were evacuated on Tuesday morning due to a nearby forest blaze.
Six fire appliances and two off-road vehicles are at the scene, including an incident command unit.
Residents not being evacuated have been advised to keep all windows and doors closed.
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Stuart Millington, a senior fire safety manager, said: "Although the cause of this fire is unknown, we are urging everybody to take extra care when they are out and about in grass and gorse land to help reduce the risk of fire, especially during the current spell of hot and dry weather.
"During drier periods, fires involving grass, bracken and heather can develop very quickly, particularly in raised winds, resulting in fires getting out of control and spreading to neighbouring properties or forestry."
He added: "We are liaising with colleagues from North Wales Police and Gwynedd County Council and once the fire is under control we will begin a full investigation to discover the cause of the fire."
Additional reporting by PA
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