Upon infection with the cecum-dwelling nematode Trichuris muris, the majority of inbred strains of mice launch a Th2-type immune response and in doing so expel the parasite before patency. In contrast, there are a few mouse strains which develop a nonprotective Th1-type response resulting in a chronic infection and the presence of adult worms. Of the Th2 cytokines known to be associated with the resistant phenotype (interleukin-4 [IL-4], IL-5, IL-9, and IL-13), comparatively little is known about the contribution that IL-9 makes towards the protective immune response. In this study we demonstrate that IL-9 is expressed early during the Th2-type response and that its elevation in vivo results in the enhancement of intestinal mastocytosis and the production of both the immunoglobulin E (IgE) and IgG1 isotypes. In addition, elevated IL-9 levels in vivo facilitated the loss of T. muris from the intestine. That IL-9 is important in promoting worm expulsion was also seen following infection of IL-9-transgenic mice, which constitutively overexpress the cytokine. These animals displayed an extremely rapid, but immune mediated, expulsion of the parasite. Also evident in these animals was a pronounced intestinal mastocytosis, which was previously shown by us to be responsible for the expulsion of the related nematode Trichinella spiralis from these animals. Taken together with observations of IL-9 production following infection with other helminths, the results imply that IL-9 contributes to the general mast cell and IgE response characteristic of these infections and, more specifically, enhances resistance to T. muris.