Residents of the Japanese city of Iwakuni have voted overwhelmingly against the expansion of a local US military base, according to exit polls.
The relocation would add 57 planes to the Iwakuni base
The vote is not binding on the Japanese government, but could put pressure on a deal already agreed between Tokyo and Washington, correspondents say.
Officials said with nearly half the ballots counted, they were 7-1 against the plan.
The redeployment is part of a project to reduce US military numbers in Japan.
The US proposes relocating a naval air wing, or 57 planes and 1,600 personnel, to its base in Iwakuni, a city 720km (450 miles) south-west of Tokyo.
But the plan has become a focus of local opposition to US forces in Japan.
There are between 40,000 and 50,000 US personnel in Japan, but the Pentagon plans to redeploy them as part of its plans to streamline its overseas bases.
According to an exit poll by Kyodo News Agency, about 90% of those who voted in the referendum rejected the plan for Iwakuni.
Organisers said at least 50% turnout was needed to validate the vote. One official said about 58% had voted.
The referendum was called by city mayor Katsusuke Ihara, who has consistently opposed the expansion of the base.
"Just because the national government has decided something doesn't mean we must refrain from expressing our views," he said.
Local concerns include the noise level at the base.
The issue of US troops in Japan is also sensitive because of a case in 1995 in which three American servicemen raped a Japanese schoolgirl in Okinawa.
Nearly 2,000 residents of another city, Zama, protested on Saturday at plans to boost US troop numbers there.
About 300 troops would be added to the 1,150 already stationed in the city just south of Tokyo.