More than 700 fire ants have been found at Nagoya, the Environment Ministry said Thursday.
No human injuries from the venomous exotic invasive pest have been reported.
The ants, all worker ants, were found moving into and out of cracks on sidewalks and holes in the ground at the port.
The ministry believes that the ants were not born from queens, but arrived from overseas, because they were found at a port and no queen ants have been spotted.
Work is underway to exterminate the ants with insecticide.