Red fire ants were found late last week in a container yard at the port of Kobe after some first turned up in a container that had passed through the port last month, the Environment Ministry said.
Red fire ants are native to South America, and about 100 of them were found at the port, the ministry said Sunday.
The Kobe Municipal Government said there is no need for concern as the nearest residential area is about 2 km away, and the city will keep looking and exterminate them if they find any more.
"As those found were worker ants, it is unlikely that an ant colony has been created with the presence of a queen," a Kobe official said.
Fire ants are an aggressive species and measure from 2.5 to 6 mm in length. People can develop anaphylaxis when stung.
The container arrived May 20 from Guangzhou, China, and was stored at the port until May 25, the ministry said. The fire ants were first found when the container was unloaded the next day in Amagasaki, Hyogo Prefecture, and were exterminated.
The ministry searched the container yard Friday and found fire ants in a crack in the asphalt. Officials spread pesticide in the surrounding areas and were searching to see if there are more.