The swarm went after and stung 76 people.
The fair was in full swing again Saturday and officials said the wasp problem had been taken care of. Two hives were removed from a palm tree next to the racetrack grandstand. Officials think the insects may have been jolted into a frenzy by vibrations during the start of Friday night's fireworks show.
On Saturday, the wasps still hovered where their nest used to be.
"I was actually right next to the tree and when the fireworks went off, I could start to see the bees come out of the tree," April Mitchell told ABC7. "One by one I could see a few people start to say hey, doing their hair and things like that."
Mitchell is the marketing manager for the fairgrounds. She was fine but her husband, son, aunt and uncle were all stung.
"It was relatively calm. For maybe the first couple minutes it became a little chaotic, but when people calmed down, they were just escorted over there. They got ice," she said.
Medics used icepacks to treat 76 people who were stung multiple times.
"They were stung all over their body," said Alameda County Assistant Fire Chief Rob Schnepp. "There was really not one place we could pinpoint and it was typical bee sting sorts of things you'd see, with localized swelling and redness."
Friday night's commotion followed Thursday's accident on the Wacky Worm rollercoaster which injured seven people.
As for the swarm of insects, fair officials say the trouble came from an unseen yellow jacket hive buried at the base of a palm tree and a paper wasp hive about 20 feet up. Maintenance workers say the one up in the tree had been there for about five years, but had never caused any trouble.
Now, it will not cause any more.
"We brought in professionals last night who, this is what they do, and this morning the fair's been open and we're all very, very confident that there are no more bees left in that area," Mitchell said.
There are no more fireworks shows planned at the fairgrounds. Maintenance workers are inspecting other trees in areas frequently populated by people to make sure there are no other hives present.