Fire crews have been mobilised to the area which covers 80 acres at the A830, south of Mallaig.
The National Trust for Scotland said the Ben Lomond fire was 1km long and was blocking the peak's Ptarmigan path.
The Mallaig fire affected grass and gorse between the A830 and B8008.
A spokesperson for the Scottish Fire and Rescue Service said: "Operations control mobilised five appliances to the scene, where crews remain as they work to extinguish two fire fronts covering an area of approximately 80 acres.
"There are no reported casualties at this time."
The National Trust for Scotland and Loch Lomond and The Trossachs National Park have asked visitors to avoid the area around Ben Lomond.
The fire comes following a extreme wildfire warning for much of the west and north east Scotland.
Warning levels for the rest of Scotland have been put at high to very high.