Schistosomiasis is a disease caused by a parasitic worm transmitted to humans by certain species of freshwater snails. In spite of several decades of intensive coordinated control schistosomiasis still infects around 1 million people in China. In order to understand the potential for spread of the disease into new areas and new snail species, it is helpful to know if the snails and parasites in China are coevolved; this means that evolutionary divergence in one group (the snails) is matched by a corresponding divergence in the other (the parasites), which is what would be expected if the two groups are locked in an evolutionary arms race. DNA-sequence data were collected for snails and parasites from the same localities. The findings indicated that coevolution was unlikely to have occurred. The implications of this are that host-switching or acquisition is more likely than previously thought. Consequently, there is a greater potential for spread of the parasite into new areas. The role of mountain barriers in confining schistosomiasis to certain regions was highlighted; this is important in view of the current plans to breach these barriers by road and rail construction that will link China and Southeast Asia.