Technical Abstract: Anthropogenic rises in atmospheric [CO2] are predicted to affect C3 and C4 weed interference with C3 crop species differently, such that C3 weeds benefit more under elevated [CO2] (eCO2), compared to C3 and C4 performance under ambient CO2 (aCO2). Our aim was to quantify impacts of eCO2 on C3 and C4 weeds at three levels of biological organization (individual, population and community) in the context of field-grown soybean (Glycine max) under FACE conditions. We conducted a field study in 2007 and 2008 within the SoyFACE experiment in Champaign, Illinois, USA, in which common waterhemp (C4, Amaranthus rudis) and common lambsquarters (C3, Chenopodium album) were grown in competition with soybean. Measurements on individuals included growth rate, biomass, height, photosynthetic rate and water use efficiency. Demographic measurements included seedling recruitment, plant survival and fecundity. Community metrics included A. rudis and C. album interference with soybean and community composition of residual weeds (ambient weeds surviving control to reach reproductive maturity). Elevated [CO2] influenced weeds at all levels of biological organization, however community level impacts were largest. Interference by A. rudis and C. album with soybean was 37% and 11% lower, respectively, in eCO2. In residual weed communities under aCO2, C3 and C4 species were equally likely to dominate the community, whereas in eCO2, there was a 90% chance that the community would be dominated by C3 species. Future investigations of weed ecology and global change under FACE conditions will improve their inference space by including sources of environmental stress such as ozone, heat and drought.