The relative competitive abilities of yellow starthistle accessions that are resistant (R) and susceptible (S) to picloram were compared using a replacement series experiment. With no herbicide treatment, total shoot dry weights at vegetative and early reproductive stages of plant growth were similar for the two accessions, although S plants accumulated more total shoot dry weight by the late reproductive stage, mainly as a result of a greater contribution of vegetative growth. Without herbicide, relative yield of total biomass or reproductive structures did not differ from theoretical competitive equivalence at any accession ratio, thereby indicating that interaccession interference was similar. For picloram-treated plants, R plants accumulated more total, vegetative, and reproductive dry weight than did S plants at the early and late reproductive stages, and there was no difference between S and R plants at the vegetative growth stage. Seed production by R plants was 10-fold greater than that observed in S plants, but seed size remained unchanged, regardless of accession ratio. With herbicide present, the relative yield of S plants differed from theoretical competitive equivalence as S:R accession ratios decreased, but relative yield of R plants did not. Therefore, only in the presence of picloram will R plants have a competitive advantage over S plants. Some of the progeny from mixed populations of S and R plants that were cross-pollinated, even at low R frequency (25%), expressed resistance to picloram.