Baylisascaris procyonis is an intestinal parasite of raccoons (Procyon lotor) that can also infect humans and a wide range of wildlife species. Prevalence of B. procyonis in raccoon populations appears to decrease as the landscape urbanizes, but less is known about prevalence in the small-mammal intermediate hosts of the parasite. We measured prevalence of B. procyonis in populations of mice (Peromyscus spp.) in forest preserves along a gradient of urbanization in Illinois. Prevalence in the mouse intermediate host exhibited a trend opposite raccoons: prevalence increased as surrounding human population density increased. This counterintuitive result may be due to higher overall environmental loads of B. procyonis in urban areas due to higher raccoon densities. Our results emphasize the need to understand fully the transmission dynamics of B. procyonis in its definitive and intermediate hosts in order to design and implement effective strategies to mitigate zoonotic risks to humans.