Organizers said this year's contestants are some of the best yet, with past champions and first-timers battling it out for the title.
As a new twist, the audience will join in the judging by voting via cellphone.
The World's Ugliest Dog contest begins at 6 p.m.
At 8 p.m., fairgoers can catch The Charlie Daniels Band, a Grammy-winning country band, at the Bud Light Stage.
The fair runs through Sunday at 175 Fairgrounds Drive in Petaluma.