The Teamsters union members, who deliver organic produce to area grocery stores, said they don't make nearly as much money as their counterparts at other companies.
The company's contract with the drivers expired in May, but the drivers kept working while negotiations were under way. The union said members rejected United Natural Foods' "last, best offer" over the weekend.
"At the point that the company refused to honor their contract, they decided that it was time to go out on strike and see if they could get them to come back to the table and bargain a fair contract," said Brenda Weist with Teamsters Local 117.
In a written statement, the company said it was disappointed with the union's decision and that its latest offer included a 10.8 percent wage increase over three years.
"UNFI has made numerous concessions during the months of negotiation and believes that the latest offer it presented to the union is fair, reasonable and competitive," the statement said.
The company said it would be hiring replacement workers to continue making deliveries.
Several dozen striking drivers were at the United Natural Foods distribution center in Auburn early Tuesday. Police were on scene, but have not had to intervene.
United Natural Foods supplies organic foods to independent markets and large grocery store chains, including Whole Foods, PCC, Metropolitan Market, Safeway, Fred Meyer, QFC and Albertsons.