Construction worker Douglas Queen died after disturbing a hive of bees on an underground sprinkler control.
His co-worker David Wolf told CBS Los Angeles: "He was on his knees, just had his hands and his face covered up... and the bees were just above him swarming.
"One of our guys went over to help him out and took his vest off and was swatting at them and that was it ... the bees were chasing him down, he ran to his car.
"They turned the water truck on to disperse the bees and get them off him - it was insane, it all happened pretty quickly. They just kept splitting up and chasing all the other workers."
Another worker on the scene said panic spread through the workers. "People were running everywhere," said Blaine Dyer.
The injured man was taken to hospital still conscious and talking. But he died later after having an allergic reaction, according to Riverside Battalion Chief Tony Perna said.
Coroner's officials later identified the victim as 49-year-old Douglas Queen of Los Angeles.
Two other men working on the construction site of a supermarket were also stung. One was treated at the scene while the other was taken to hospital for treatment and is believed to be well enough to return to work at another site.
"There were bees all over them. Hundreds of them," Tomace Marquez told the Press-Enterprise.
Honey bee stings release pheromones that prompt other nearby bees to attack. According to US government statistics, about 3.3% of adults will experience anaphylaxis after an insect sting.
There are up to 100 deaths per year from insect-sting-related anaphylaxis. Anaphylaxis is a severe type I hypersensitivity allergic reaction in humans.