The union’s annual report, filed Thursday with the Labor Department, showed that membership grew 6 percent, or 21,421, to 376,612 in 2010. The union’s 2009 membership of 355,191 was the lowest since 1940, five years after it was founded and the year before it organized workers at the Ford Motor Company.

General Motors, Ford and the Chrysler Group each created thousands of jobs last year as they ramped up production and increased their sales from the dismal levels reached during the recession. The U.A.W. also has worked to compensate for its losses in the auto industry by organizing employees of casinos and other businesses outside of manufacturing.

“This increase is a reflection of new organizing by the U.A.W., the recovery of the domestic auto industry and U.A.W. members who won a first contract during the year,” Bob King, the U.A.W.’s president, said in a statement. “We hope to continue this growth in 2011 and beyond, as we fight to win a more fair and democratic process for workers to organize unions in the United States.”

Ford has committed to adding 7,000 hourly and salaried jobs in 2011 and 2012, and G.M. executives said this week that more workers probably would be needed this year.

The union’s new members amount to just a tiny fraction of the number it has lost since peaking at more than 1.5 million in 1969, according to union data. It still had more than 700,000 members 10 years ago.

Mr. King, who was elected last year, is seeking to increase the union’s influence by redoubling longstanding efforts to organize workers at the plants operated by foreign carmakers in the United States. He plans to identify soon which company those efforts will focus on first.

Another top U.A.W. official said this week that encouraging the Detroit automakers to create jobs would be a priority during contract negotiations this fall.

“They could get the best contract imaginable and that’s completely undermined if jobs continue to move offshore,” Harley Shaiken, a professor at the University of California, Berkeley who specializes in labor relations, said.

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