Background Ascaris lumbricoides and Ascaris suum are nematode parasites that infect millions of people and pigs worldwide, respectively. Reports of cross-infection between the two host species has stimulated molecular epidemiological studies of the Ascaris genus. In this study, we evaluated the dynamics of Ascaris transmission between Guarani indigenous schoolchildren, pigs, and the environment of a village in the state of Paraná, southern Brazil. Methodology/Principal findings Parasitological and molecular analyses of fecal samples from humans and pigs, and soil samples from the village were carried out. Eggs of Ascaris spp. were observed in 8.4% (7/83) of human samples, 44.4% (8/18) of pig samples, and 8.9% (6/68) of soil samples. PCR amplification of the ITS-1 locus of the rDNA gene in samples that were positive in the parasitological examination revealed cross-infection by the two species, A. lumbricoides and A. suum, in human and swine hosts. The soil, which was contaminated by both human and swine feces, also contained eggs of the two Ascaris species, thus constituting a source of Ascaris infection for both hosts. DNA from both nematode species, individually and mixed, was detected in samples from both hosts and the soil. Conclusions/Significance The results of this study indicate that more effective control measures, aimed at the correct disposal of both human and animal feces, should be implemented.