Lepidium draba L. (Brassicaceae) is an exotic perennial herb indigenous to Eurasia that successfully established in North America in the late 1800’s. The niche saturation hypothesis predicts that in the indigenous range, long coevolved insect-plant relationships will result in niche saturation, and plants introduced to exotic ranges will 1) primarily be utilized by polyphagous species and 2) will be comprised of vacant niches. In 2002 and 2003, the insect community associated with L. draba was surveyed in the northwestern United States, and in its indigenous range in eastern Europe. The diversity of the insect community in the indigenous range was greater for both species richness and evenness. The community structure was predominated by polyphagous species in the introduced range and specialists in the indigenous range. Total insect abundance was greater in the introduced range, but differences in the insect diversity between origins was most strongly influenced by one dominant polyphagous herbivore, Lygus elisus Van Duzee, which represented over 50% of the total insect abundance in the northwestern U.S. However, one indigenous American species, Ceutorhynchus americanus Buchanan, was found utilizing the stems of L. draba, which may demonstrate a host shift to a specialized niche on a related exotic plant. Guild analyses indicate that unoccupied niches exist in the introduced range, whereas niche saturation by specialist herbivores is evident in the indigenous range. Our data corroborate well with the niche saturation hypothesis and provide one mechanism that may be used to explain the invasiveness of L. draba in the northwestern U.S.