Studies were conducted to determine the growth, fecundity, and competitive ability of an acetyl-CoA carboxylase (ACCase)–inhibitor resistant (R) sterile wild oat biotype compared with a susceptible (S) biotype. Seed germination studies indicated that there were no differences in seed germination and seedling vigor between R and S biotypes at any temperature regime. R and S biotypes were grown under noncompetitive and competitive arrangement in the greenhouse. Under noncompetitive greenhouse conditions, growth of the R biotype was similar to that of the S biotype on the basis of plant height, canopy area, and plant biomass. Seed production and weight of R and S plants were also at the same levels. Furthermore, relative competitiveness among the R and S sterile wild oat biotypes was investigated by means of replacement series experiments. The R and S biotypes were compared under seven mixture proportions (6 : 0, 5 : 1, 4 : 2, 3 : 3, 2 : 4, 1 : 5, and 0 : 6). No significant differences in competitive ability were observed between R and S biotypes on the basis of plant height, canopy area, or plant biomass. In most cases, relative crowding coefficient (RCC) values at 20, 60, and 100 d after transplanting (DAT) were close to one, indicating equal competitiveness between the R and S biotypes of wild oat used in this competitive study. However, in some cases, the RCC value was 1.31 for plant height, evident of a slight competitive advantage for the R biotype at 100 DAT. In general, ACCase-inhibitor R and S sterile wild oat biotypes were equally competitive, clearly without any growth penalty for R plants in either noncompetitive or competitive conditions.