Flight attendants from the merged United and Continental airlines picked the union from United to represent their combined group on Wednesday.
The Association of Flight Attendants-CWA won the election, ousting the International Association of Machinists and Aerospace Workers. The AFA said it got 55.1 percent of the vote.
The voting included 15,159 United flight attendants who are already in the AFA. That union now gains almost 9,300 flight attendants from Continental.
United Continental Holdings Inc. has run both airlines since last year and is combining them into a single carrier flying under the United name.
United Continental will now need to negotiate a single contract to cover the combined group. In a statement, the airline called the vote "another important step forward in our integration process."
"We look forward to a constructive working relationship with the Association of Flight Attendants," the airline said.
Greg Davidowitch, president of the AFA chapter at United, said in a statement that the union will "maximize our leverage to set the highest standards at the world's leading airline. Flight Attendants are dedicated to ensuring a successful merger and fully participating in its benefits."
The vote bolsters the AFA, which has not been able to keep its foot in the door at Delta Air Lines Inc. The AFA represented Northwest Airlines flight attendants but narrowly lost a vote to cover the combined group after Delta bought Northwest in 2008. The National Mediation Board is looking into allegations of interference and could order a new vote.
Shares of Chicago-based United Continental fell 37 cents to $22.91 in afternoon trading.