We used an individual-based simulation model (ECOTONE) in conjunction with climate and soil texture data in a GIS to identify grassland sites in Colorado where the invasive perennial forb, Acroptilon repens (Russian knapweed), is likely to be successful. We also created a threat map indicating the most vulnerable regions for future invasion within the state. This species is a serious problem throughout western rangelands of the U.S. and Canada. Our results show that A. repens has the potential to become most abundant in dry areas with fine-textured soils. Validation of model results with existing infested sites shows that additional areas are at risk from invasion. This information can be used to direct management efforts towards the areas at greatest risk, allowing the most effective use of limited resources.