Technical Abstract: Studies were conducted to determine the relative fitness and competitive ability of an acetolactate synthase (ALS) inhibitor–resistant (R) downy brome biotype compared with a susceptible (S) biotype. In previous research, the mechanism of resistance was determined to be an altered ALS enzyme. Seed germination of the R biotype was compared with that of the S biotype at 5, 15, and 25 C. There were no different germination characteristics between R and S biotypes at 15 and 25 C. However, the R biotype germinated 27 h earlier than the S biotype and had reached over 60% germination when the S biotype initially germinated at 5 C. Under noncompetitive greenhouse conditions, growth of the R biotype was similar to that of the S biotype on the basis of shoot dry weight, leaf area, and plant height. Seed production of the R biotype was 83%, when compared with the S biotype, but seeds of the R biotype were larger than those of the S biotype. Replacement series experiments were conducted in the greenhouse to determine the relative competitive ability of R and S biotypes. No difference in competitive ability was observed between R and S biotypes on the basis of shoot dry weight, leaf area, or plant height. Thus, it appears that ALS-resistance trait is not associated with growth penalty in either noncompetitive or competitive conditions. In the absence of ALS inhibitors, these results suggest that the R biotype would remain at a similar frequency in a population of R and S biotypes.