The economic impact of three invasive, exotic weeds--diffuse, spotted, and Russian knapweed (Centaurea diffusa, C. maculosa, and Acroptilon repens)--on Montana's economy was estimated using a procedure developed for another invasive weed species. Published data and that from a survey of county weed boards were used to estimate direct negative impacts of over $14 million annually due to infestation of over 2 million acres of rangeland and wildland. This amounts to about $10.63 on each infested grazing land acre and $3.95 on each infested wildland acre. Direct plus secondary economic impacts, estimated using an input-output model, are about $42 million annually, which could support over 500 jobs in the state's economy. This first approximation suggests the knapweed infestation problem in Montana deserves attention, although more work could be done to refine these estimates and to allow estimation of the impacts at sub-state levels.