Over 90% of Canadian kochia populations are resistant to acetolactatesynthase (ALS)– inhibiting herbicides. We questioned whether the targetsite–based resistance could affect plant growth and competitiveness.Homozygous F2 herbicide-resistant (HR) kochia plants with anamino acid substitution at Trp574 (sources: Alberta [AB],Saskatchewan [SK], and Manitoba [MB]), or Pro197 (MB, AB with twopopulations) were grown in replacement series with homozygous F2herbicide-susceptible (HS) plants from the corresponding heterogeneouspopulation (total: six populations). In pure stands, growth of HR plantsfrom AB and SK was similar to that of HS plants, regardless of mutation;conversely, MB2-HR plants (Trp574Leu) developed more slowly andwere taller than MB2-HS plants. Final dry weight of HR plants in pure standswas similar across all six populations, whereas that for HS plants in purestands and HR–HS plants in mixed stands (50–50%) varied with population.Results for AB and SK populations suggest little impact of either ALSmutation on kochia growth, whereas those for MB lines would suggest anunidentified factor (or factors) affecting the HS, HR, or both biotypes. Thevariable response within and between lines, and across HS biotypeshighlights the importance of including populations of various origins andmultiple susceptible controls in HR biotype studies.