Black henbane (Hyoscyamus niger L.) is a poisonous, invasive plant in the family Solanaceae.  Henbane is typically associated with highly disturbed situations such as pipelines, roadsides and mammalian burrows.  Recent field research has determined that henbane is unable to invade or survive in close proximity to established rangeland grasses, but upon removal of the vegetative competition, henbane is successful.  A greenhouse study was conducted to determine if there were differences between henbane growth alone or in combination with three common Northern Mixed Prairie grass species.  Species utilized were Western wheatgrass (Pascopyron smithii), Idaho fescue (Festuca idahoensis) and Sandberg bluegrass (Poa secunda), both as mature plants and seedlings.  Our results indicate that while there were no differences between mature grasses grown alone and in combination with black henbane, all measures of henbane growth were significantly higher among plants grown without a mature grass pot companion.  Henbane in combination with the same grass species as seedlings exhibited reductions in biomass and rosette diameter, but not root diameter or leaf production.  Idaho fescue seedlings were not negatively impacted by henbane seedling presence, while tiller number and biomass were reduced for Western wheatgrass and Sandberg bluegrass, respectively.  These results suggest that henbane is not well suited to invasion of mature grass stands and will not likely become major problem in undisturbed rangeland; however, in disturbed situations, henbane is better able to compete with some newly emerging native grasses.