The Scottish Fire and Rescue Service sent 15 appliances to the scene on Livingston’s Katherine Street on Thursday after being alerted at around 8.30pm on Thursday night.
It is understood that they are still working on tackling the blaze on Friday morning.
The service said the blaze erupted in a ground floor flat in a four-storey building.
A Scottish Fire and Rescue Service said: “We were alerted at 8.35pm on Thursday, January 27 to reports of a fire within a ground floor flat in a four-storey building on Katherine Street in Livingston.
“Operations controls mobilised 15 appliances, three height appliances and specialist resources at the scene.
“Firefighters are working on controlling the fire and are currently at the scene.”
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Police Scotland also attended and it is understood that all residents were evacuated safely.
Road closures remain in place on the street and surrounding area.
A Police Scotland spokesperson said: “Police Scotland and the Scottish Fire and Rescue Service are currently dealing with fire in Katherine Street, Livingston.
“All residents have been evacuated safety.
“There are currently road closures in place whilst we deal with this incident. We thank you for your patience.”