Of morphologically indistinguishable small-sized neodiplostomula in the grass snake Rhabdophis tigrina in the Republic of Korea, some, such as Neodiplostomum seoulense, mature into adults in the intestines of rodents, whereas others, such as Neodiplostomum leei, migrate to rodent livers and mature in the intestines of chicks. In the present study, we aimed to observe in more detail the extraintestinal migration and development of N. leei by using various animal models, i.e., mice, rats, hamsters, rabbits, cats, and chicks. In mammals, small-sized neodiplostomula (N. leei) inoculated orally penetrated the intestinal wall, entered the peritoneal cavity, and oriented to the liver without passing through any other organ. In rodent livers, the neodiplostomula were surrounded by host inflammatory cells and fibrotic tissues from day 10 postinfection (PI); the worms were found dead by day 56 PI. When neodiplostomula from rodent livers were transferred orally to mammals, they reoriented to the liver, although they were able to develop into adults in the chick intestine by day 6–7 PI. The possibility of human infections, i.e., liver migration, by N. leei neodiplostomula, among snake consumers, warrants investigation.