Human ascariasis is caused by infection with the common roundworm Ascaris lumbricoides, although the pig roundworm Ascaris suum has also been reported to infect humans and develop into the adult stage. To elucidate whether pig-derived Ascaris infects humans in Japan, 9 Ascaris isolates obtained from Japanese patients and a further 9 Ascaris isolates of pig origin were analyzed to determine their internal transcribed spacer-1 sequences. Six of the 9 clinical isolates showed the Ascaris genotype which predominantly infects humans in endemic countries, while the other 3 clinical isolates and 9 pig-derived isolates showed the genotype predominant in pigs worldwide. These results suggest that at least some cases of human ascariasis in Japan are a result of infection with pig-derived Ascaris.