The vote came after the United States, Mexico and Canada revised a trade agreement that sought to strengthen the hand of workers.

Workers at one of the largest General Motors plants in Mexico voted to adopt an independent union on Thursday in what was seen as an important test case for whether new North American trade rules can improve working conditions and stamp out corruption in the Mexican labor system.

The union, called the Independent National Autoworkers Union, won 78 percent of the votes cast at the plant in Silao, where more than 6,000 workers assemble Chevy Silverados and G.M.C. Sierra pickup trucks. The vote pushed out the Confederation of Mexican Workers, which had held the contract for the last 25 years.

Workers at the Silao plant start out earning less than $9 a day, and have described punishing working conditions. Employees have said they are often denied breaks and are rarely offered raises.