Dozens of fire ants have been found and exterminated at Aomi Pier on Tokyo Bay.
Officials with the Environment Ministry and the Tokyo Metropolitan Government say a pier worker was bitten on the hand by ants when he picked up straw near the entrance of a container yard on Friday of last week.
Port workers found dozens of the insects in the area and exterminated them on the spot.
On Tuesday, experts identified the ants as highly poisonous fire ants, which are native to South America.
Ministry officials say the worker was slightly injured by the bite.
The Tokyo government surveys the port area twice a year. Officials say no fire ants were found in the latest survey in late May.
Fire ants become active in hot weather. Ministry officials are calling on people to report to counseling centers if they find any strange ants, without touching them.