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Cheatgrass (Bromus tectorum L.) and yellow starthistle (Centaurea solstitialis L.) have invaded over 250 thousand hectares throughout the Pacific Northwest. Future management of rangelands dominated by these species will require an understanding of the plant-plant interactions which contribute to the regulation of community dynamics and the establishment of desirable plants. Addition series experiments, with total stand densities ranging from 20-20,000 plants m(-2), were used to quantify the interference between cheatgrass and yellow starthistle in unrestricted soil depths on 12-day intervals throughout the growing season and in soil depths restricted to 0.2- and 0.5-m. Intraspecific interference was nearly twice as important as interspecific interference when plants were grown in unrestricted soil. Resource partitioning by cheatgrass and yellow starthistle was associated with rooting depth. When restricted to a 0.5-m depth, resource partitioning did not occur and intra- and interspecific interference were similar for both species. Restriction to a 0.2-m depth resulted in cheatgrass density being twice as important as yellow starthistle density for predicting yellow starthistle shoot weight. Yellow starthistle density was not important for predicting cheatgrass shoot weight. Cheatgrass appeared to have a competitive advantage over yellow starthistle in shallow soils, but the rooting depth and seed production capacity of yellow starthistle was greater than cheatgrass in deep soil. The Journal of Range Management archives are made available by the Society for Range Management and the University of Arizona Libraries. Contact lbry-journals@email.arizona.edu for further information. Migrated from OJS platform August 2020