Hundreds of highly poisonous fire ants have been found at the port of Yokohama, near Tokyo.
Officials of the Environment Ministry found more than 500 ants near containers at a wharf on Friday.
The officials say experts later confirmed them to be fire ants.
They say the survey also found larvae and pupae -- each numbering more than 100 -- and this suggests that the fire ants are reproducing at the wharf.
The ministry will investigate further.
Hundreds of fire ants were found for the first time in Japan in May. They were spotted in a container in Amagasaki, Hyogo Prefecture in western Japan. The container from China was transported there after being unloaded at the nearby port of Kobe.
Since then, the species has been found at a number of places in Japan.