The Scottish Fire and Rescue Service (SFRS) was called to reports of a wildfire at Market Stance, Balivanich, on the Isle of Benbecula, at around 3.30pm on Wednesday.
Three appliances attended the scene, and the fire was said to be about 1,000 metres wide.
The SFRS has scaled it back to around 80 metres, and remains on scene.
On the Isle of Harris, the fire service was alerted to a wildfire just after 6.30am on Thursday at Horsacleit.
The SFRS sent two appliances to the 1,600-metre-wide blaze, and remains in attendance.
There have been no reported casualties at either site.
SFRS area commander Bruce Farquharson warned on Wednesday that despite the freezing temperatures, parts of the west coast are vulnerable to wildfires.
He said: “There may have been a lot of snow in the eastern and central areas of Scotland, but that is not the case in the western coastal areas and fuel conditions are very different.
“At this time of year, we typically have a large volume of dead, bone-dry vegetation left over – which essentially acts as a fuel for fire.
“As a result, there are currently vast areas of countryside all over the country that are tinder dry and vulnerable, this provides all of the ingredients for fire to take hold and spread.
“We are asking the public to exercise extreme caution and think twice before using anything involving a naked flame.”