The hotel, which has nearly 4,000 rooms, is one of the largest in the world.
The blaze was reported at 10:46 p.m. on a building at the southside of the hotel's lake and 77 firefighters responded, knocking it down by 11:09 p.m.
The Las Vegas Strip was closed to traffic near the casino as firefighters battled the flames.
There were no immediate reports of injuries.
The cause is being investigated.
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LAS VEGAS (KTNV) - A fire has been reported at Bellagio hotel-casino Thursday night.
Clark County Fire Department first responded to the fire at 10:50 p.m. after receiving the first call at 10:46 p.m. The fire was located on the roof of a building on the north side of the lake.
Firefighters reported at 11:09 p.m. that fire was knocked down. Clark County Fire said firefighting efforts were "extremely difficult" due to the location of the fire and access to the location.
A total of 77 personnel responded to the scene. A second fire was called as a precaution.
There are no reported injuries.
Las Vegas Boulevard is closed as a result of the fire.