The video shows a man strolling the aisles of the convenience store and talking with the clerk.
Then it shows him walking over to her and repeatedly punching her.
Later in the video, the 50-year-old attendant gets down on the ground and the man leaves.
West Covina Police Corporal Rudy Lopez said police need help identifying the assailant, who took a large sum of cash from the register.
"At this point, we don't have any leads," he said.
The man is white, between 30 and 40 years old and between 5' 8" and 5' 10," Lopez said.
At the time of the alleged assault and robbery, he had blond hair, a mustache and a goatee.
He was wearing a baseball cap, dark-colored short-sleeve shirt, light pants and sunglasses.
(Copyright © 2012 NBC Universal, All Rights Reserved)