Studies were conducted to determine the growth, fecundity, and competitiveability of an acetyl-CoA carboxylase (ACCase)–inhibitor resistant (R)sterile wild oat biotype compared with a susceptible (S) biotype. Seedgermination studies indicated that there were no differences in seedgermination and seedling vigor between R and S biotypes at any temperatureregime. R and S biotypes were grown under noncompetitive and competitivearrangement in the greenhouse. Under noncompetitive greenhouse conditions,growth of the R biotype was similar to that of the S biotype on the basis ofplant height, canopy area, and plant biomass. Seed production and weight ofR and S plants were also at the same levels. Furthermore, relativecompetitiveness among the R and S sterile wild oat biotypes was investigatedby means of replacement series experiments. The R and S biotypes werecompared 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 wereobserved between R and S biotypes on the basis of plant height, canopy area,or plant biomass. In most cases, relative crowding coefficient (RCC) valuesat 20, 60, and 100 d after transplanting (DAT) were close to one, indicatingequal competitiveness between the R and S biotypes of wild oat used in thiscompetitive study. However, in some cases, the RCC value was 1.31 for plantheight, evident of a slight competitive advantage for the R biotype at 100DAT. In general, ACCase-inhibitor R and S sterile wild oat biotypes wereequally competitive, clearly without any growth penalty for R plants ineither noncompetitive or competitive conditions.