Andrew M. Hedberg, Victoria A. Borowicz, and Joseph E. Armstrong. (Behavior, Ecology, Evolution, and Systematics Section, Campus Box 4120—Biological Sciences, Illinois State University, Normal, IL 61790-4120). Interactions between a hemiparasitic plant, Pedicularis canadensis L. (Orobanchaceae), and members of a tallgrass prairie community. J. Torrey Bot. Soc. 132: 401–410. 2005.—Pedicularis canadensis (lousewort) is a hemiparasitic plant of open woodland and prairie habitats. The interactions between this hemiparasitic plant and selected members of a tallgrass prairie community were investigated via a greenhouse experiment and field observations in a restored prairie. The greenhouse experiment examined interactions between P. canadensis and hosts from each of three families prominent in prairie communities: big bluestem (Andropogon gerardii, Poaceae), tall goldenrod (Solidago canadensis, Asteraceae), and showy tick trefoil (Desmodium canadense, Fabaceae). Individual hemiparasites were grown with one host, with two hosts of a single species, or with two hosts of different species, in all combinations. P. canadensis reduced tall goldenrod shoot mass, marginally affected big bluestem shoot mass, and had no effect on root mass of these two host species or on shoot and root mass of showy tick trefoil. The presence of the hemiparasite did not alter competitive relationships between host species and its effect was similar to that of a competitor. Spearman rank correlations were calculated between P. canadensis cover and species richness and abundances, including bare ground, for Six-Mile Creek Prairie, a restored tallgrass prairie in McLean County, Illinois. Species richness was positively correlated with increasing P. canadensis cover. These results suggest P. canadensis can influence prairie community composition.