Zahrobska, the surprise bronze medalist at the last worlds in Bormio, Italy, in 2005, led after the first run and protected her lead the second time down the floodlit Gastrappet course to win in a combined time of 1 minute, 43.91 seconds.
No Czech skier other than Zahrobska has won a world championship medal, and none has won an Olympic medal, in an Alpine event.
"I was a little bit nervous in the start hut, but all the girls were," she said. "Before the second run I felt good for a podium, but not for gold."
Austria's Marlies Schild, who has won five of the six slalom races on the World Cup circuit this season, climbed from third after the opening leg into second place in 1:44.02.
Reigning Olympic champion Paerson _ who was aiming to become the first skier to win four gold medals at a single world championship _ was third in 1:44.07 after making a mistake near the finish line on the second run. The Swede won the super-giant slalom, combined and downhill last week.
"On the one hand, I'm happy to know I can compete with the fastest girls in slalom again," Paerson said. "But on the other, the mistake I made near the bottom cost me about .20 that would have meant gold."
Resi Stiegler was the top American, finishing eighth in 1:44.98. Lindsey Kildow missed the race after injuring her right knee during a training run. Olympic giant slalom champion Julia Mancuso also skipped the race due to fatigue.