Assessing belowground plant competition is complex because it is verydifficult to separate weed and crop roots from each other by physicalmethods. Alternative techniques for separating crop and weed roots from eachother are needed. This article introduces a stable isotope method that canquantify the amounts of roots of rice and barnyardgrass intermixed inflooded field soils. It relies on the biological principle that rice, a C3 (photosynthetic pathway) species, discriminates moreeffectively than barnyardgrass, a C4 species, against arelatively rare isotopic form (13C) of CO2. Thisresults in different 13C: 12C isotope ratios(expressed as δ13C) in root tissues of the two species. δ13C values for monoculture barnyardgrass and rice grown in astandard flood-irrigated system were highly stable over 4 crop-years,averaging −13.12 ± 0.80 (SD) and −28.5 ± 0.11 (SD)‰, respectively, based onanalysis by an isotope ratio mass spectrometer. Standard concentrationcurves relating measured δ13C values to set proportions ofrice:barnyardgrass root biomass were described by linear regressions,typically with r2 values of 0.96 or greater. Quantities of intermixed rice andbarnyardgrass roots sampled 0 to 5 cm deep from soil between rice rows wereestimated by extrapolation from standard curves based on δ13Cvalues. About 50% more barnyardgrass root tissue was detected in plots ofLemont long-grain rice than in weed-suppressive PI 312777 indica rice,demonstrating the feasibility of using this stable carbon isotope method inflooded rice systems.