Measure "M" calls for a ten-year ban of genetically modified crops in Sonoma County.
Traditional farmers oppose the proposition. They say eliminating biotech options could cost them millions of dollars in losses from diseases. Organic farmers claim modified crops could harm the county's reputation for natural foods.
Both sides predict the vote will be close.
Dave Kranz of the California Farm Bureau says most Central Valley farmers don't like the concept of banning biotech crops.
"Many San Joaquin Valley county, boards of supervisors, have already taken positions in favor of agriculture biotechnology."
One farmer from Yolo County says some farmers are concerned that if the ban is approved in Sonoma, it might encourage those wanting to outlaw biotech crops to get a similar initiative on the ballot in their county.
Currently three California counties have laws banning genetically engineered crops. Last November three other counties rejected initiatives, which would have prohibited biotech crops.