Background/Question/Methods Past research shows that earthworms can affect plant growth either directly or indirectly through alteration of root structure and biomass, nutrient availability, soil moisture, relative abundance of microbial functional groups, and abundance of arbuscular mycorrhizal (AM) fungi (via disruption of intra- and inter-radical hyphae). Although much research has addressed the widespread impacts of non-native earthworms on plant communities in historically earthworm-free regions, few studies have compared effects of native and non-native earthworms on plant growth in ecosystems with native earthworms, such as tallgrass prairies. We conducted a mesocosm study to experimentally assess the individual and combined effects of native and non-native earthworms on big bluestem (Andropogon gerardii), a dominant plant species in North American tallgrass prairies. Each mesocosm included a single plant and was subjected to one of the following earthworm treatments for 16 weeks: no earthworms (control), natives only, non-natives only, and both natives and non-natives. Among all treatments, we compared above- and belowground biomass, root structure, relative abundance of soil microbial functional groups, and intra- and inter-radical AM fungal abundance. Results/Conclusions Preliminary results show that neither native nor non-native earthworms individually affected aboveground plant biomass, relative abundance of soil microbial functional groups, or intra- and inter-radical AM fungal abundance. However, the combination of native and non-native earthworms was found to affect root structure, with a significant effect on fine:coarse root ratio (p-value = 0.043) and nearly-significant effect on average root diameter (p-value = 0.053). The combined native and non-native treatment had a significantly greater fine:coarse root ratio and lower average root diameter compared to native earthworms only, and no combination of native and non-native earthworms was significantly different from plants grown without earthworms. We found no significant effects of earthworms on belowground biomass or specific root length. Our findings suggest that while earthworms do not affect total plant growth or root growth, they do affect root structure and, therefore, will likely affect the ability of plants to acquire nutrients and water. More research is needed to further evaluate the separate and combined effects of native and non-native earthworms on plant growth and microbial composition in grasslads, as non-native earthworm abundances are projected to increase in these ecosystems.