Ascaris is a large parasitic roundworm (nematode) of the small intestine of humans and pigs, which causes the socio-economically important disease, ascariasis. To better understand the relationship of Ascaris between the 2 host species, recent studies in China have focused on investigating the genetics and epidemiology of Ascaris from humans and pigs using a mutation scanning-based approach. Findings provided support for a low level of gene flow between the human and porcine Ascaris populations. Extending the studies of genotypic variability within Ascaris from humans and pigs, experimental infections of mice and pigs with selected genotypes of Ascaris were carried out. Initial results indicate that there is a significant difference in the ability of Ascaris eggs of genotype G1 (derived from human) and G3 (derived from pig) to infect and establish as adults in pigs, supporting the difference in the frequencies of these genotypes in natural Ascaris populations between pigs and humans in China. Taken together, current information supports that there is limited cross-infection of Ascaris between humans and pigs in endemic regions and that pigs are not a significant reservoir of human infection with the adult nematode in such areas.