Metagonimoides oregonensis was found in nine new localities in North America and has now been reported from three Pacific Coast states, two midwestern states, and three southern states. Geographically, three separate population groups of M. oregonensis exist in North America; however, they may have been contiguous in the recent past. The morphological development of M. oregonensis from California is different from that of worms from Michigan and North Carolina. Based on sucker-acetabulum ratios, 10-day-old worms from Michigan are similar to those from North Carolina and are different from the California population, in which the acetabulum is considerably larger than the oral sucker. The oral sucker-acetabulum ratios are accompanied by differences in the intrasnail portion of the life cycles and final integration with the definitive host. This parasite matures rapidly in experimental and natural hosts and produces eggs by 6 days after infection. Worms increase in size until 10 days old. The adult phase of the life cycle is short-lived in laboratory and natural hosts.