Background/Question/Methods A large body of research has shown the importance of plant species diversity in structuring arthropod communities. More recent attention has been paid to genotypic diversity within a plant species, and analogous effects on community structure have been observed. However, no study to date has directly compared the relative importance of plant genotypic and species diversity on arthropods, or tested whether the mechanisms for community shifts with diversity are similar. To make this comparison, we propagated 24 genotypes of the common evening primrose (Oenothera biennis) and 24 common old-field species from locally collected seeds and, using a substitutive common garden design, planted them in an old-field environment. We established four treatments by arranging rings of eight plants with either low (one O. biennis genotype) or high (eight O. biennis genotypes) genotypic diversity or low (one old-field species) or high (eight old-field species) species diversity. We censused the arthropod community on every plant two times over the growing season, collected data on numerous plant traits, and scored the amount of herbivory that occurred on each plant. Results/Conclusions We found that arthropod abundance and richness increased with both plant species diversity and O. biennis genotypic diversity, and these arthropod responses were correlated with increases in plant biomass as well as a number of other plant traits. Total arthropod abundance responded similarly to both plant genotypic and species diversity, although predator abundance showed a greater increase with species diversity. Conversely, total arthropod richness showed a greater increase with plant species diversity compared to genotypic diversity, and this was true for all trophic levels. We found that arthropod diversity (Shannon index) increased with plant species diversity but actually decreased with genotypic diversity, implying that evenness of the arthropod community in genetically diverse patches of O. biennis was far lower than that in monocultures. This effect was largely driven by a single arthropod on O. biennis, Plagiognathus politus (Miridae), a generalist omnivore. This insect responded non-additively to O. biennis genetic diversity such that its increase in abundance in polycultures was three times more pronounced than the rest of the community. Overall, our results suggest that an often overlooked aspect of species interactions- intraspecific variation- can have profound effects in communities.