Fire crews were called to two fires on Bangor mountain in Gwynedd early on Sunday in the space of just 11 minutes.
Meanwhile, fire crews in mid Wales have been battling a two-mile-wide grass fire since 1130 BST on Sunday.
In the first blaze at Bangor mountain, reported at 0045 BST, 400 square metres of gorse were reported to be alight.
Then, at 0056 BST another 100 square metres were reported to be on fire on a separate part of the mountain opposite Dean Street.
Three crews tackled the blazes which North Wales Fire Service said were started deliberately.
The blaze at Elan Valley near Rhayader in Powys is being tackled by firefighters from four engines and two four-wheel drive vehicles carrying portable pumps.
The are also using fogging devices, which push water out at high pressure to create a mist over the flames.
The cause of the fire is not yet known but Mid and West Wales Fire Service said they were expecting to be there for some hours.