Proposition 2, which outlaws cramped cages for egg-laying chickens, has passed.
The Standards for Confining Farm Animals Act drew some high-profile backers, including Ellen DeGeneres and wife Portia de Rossi. Opponents argue the measure will drive egg producers out of state.
With 19 percent of the vote tallied, 62 percent of voters are supporting the measure.
The proposition was the farthest-reaching measure dealing with farm animal treatment ever put before voters in any state. It prohibits ranchers from keeping chickens, veal calves and breeding pigs in pens that are too small for them to stand up, turn around or stretch.
The restrictions will take effect in 2015. Violations will carry a $1,000 fine.