Technical Abstract: Assessing below ground plant interference in rice has been difficult in the past because thorough, accurate separation of the intertwined roots of weed and crop is extremely challenging. A d13C depletion method has been developed to assess interactions between roots of barnyardgrass and weed-suppressive rice in flooded fields, but the suitability of this approach for other rice weed species is not known. Thus, d13C (an expression of 13C:12C ratios) levels in roots and leaves of rice were compared to those of ten troublesome weed species grown in monoculture in greenhouse and/or field. Species included the C4 tropical grasses: barnyardgrass, bearded sprangletop, Amazon sprangletop, broadleaf signalgrass, fall panicum, and large crabgrass; the C4 sedge: yellow nutsedge; and the C3 weed species: red rice, gooseweed, and redstem. Rice root d13C levels averaged ~-28‰ indicating that these roots are highly 13C-depleted. Root d13C levels ranged from -12‰ to -17‰ among the tropical grass weeds, and were -10‰ in yellow nutsedge, indicating that these weed species were much less 13C-depleted than rice, were C4 plant types, and were suitable for 13C discrimination studies with rice. Among the C4 weed species, bearded sprangletop and yellow nutsedge were most and least 13C-depleted, respectively. d13C values for all species tested were strikingly consistent from year to year and in different environments. Root d13C levels in pot-grown plants averaged only 0.5‰ lower in a greenhouse than in the field. Typically, shoots of rice were more 13C-depleted than roots, whereas the reverse was true for the C4 weed species. These relationships between d13C levels in roots and shoots were consistent within species, suggesting the possibility of using shoot d13C values as a proxy for root d13C values in calibration curves if pure monoculture root standards are unavailable.