Background/Question/Methods Yellow starthistle represents one of the most spectacular examples of biological invasion in the Western United States. However, the mechanisms leading to its success have not been clearly elucidated. While its success has been attributed to superior competitive ability, few competition studies have been performed with yellow starthistle. Yellow starthistle and wild oat (an important component of California annual grasslands) were grown in monocultures and mixtures to assess the strength of competitive interactions between them. Results/Conclusions For either species, intraspecific competition exerted a greater influence over mean plant weight than did interspecific competition.  A companion study revealed partial temporal separation in the phenology of these species. Additional measurements of growth and soil moisture dynamics in large 270 x 50 cm PVC columns also showed a lack of interspecific competition and confirmed that water use patterns differed between these species, indicating niche partitioning. Wild oat reduced soil moisture to 5% but only to a depth of approximately 150 cm. Yellow starthistle depleted soil moisture to less than 5% throughout the column to a depth of at least 270 cm. These patterns were present when wild oat and yellow starthistle were grown individually or together in the columns, indicating that yellow starthistle had a greater impact on soil moisture and to greater depths. Yellow starthistle’s invasion of annual grasslands in the western U.S. does not appear to be due to superior competitive ability, but may be due to its ability to access deeper soil moisture at a time when annual grasses have senesced. These results do not support the hypothesis that invasive plant's are successful because of their superior competitive abilities.