Ganie, Z. A., Stratman, G. and Jhala, A. J. 2015. Response of selected glyphosate-resistant broadleaved weeds to premix of fluthiacet-methyl and mesotrione (Solstice™) applied at two growth stages. Can. J. Plant Sci. 95: 861-869. A premix of fluthiacet-methyl and mesotrione (1:17.5 ratio) has recently been registered for post-emergence (POST) broadleaf weed control in corn. The objective of this study was to determine the response of glyphosate-resistant common waterhemp, giant ragweed, and kochia to a premix of fluthiacet-methyl and mesotrione when applied to 10- and 20-cm-tall plants. Greenhouse dose response studies were conducted and log-logistic models were used to determine how the response varies between the weed species at two growth stages under various rates ranging from 21.8 to 218.5 g a.i. ha^-1 (0.25× to 2.5×). The effective rate required to achieve >90% control and shoot biomass reduction varied depending on the weed species and growth stage. The rates required for 90% control (ED₉₀) of 10-cm-tall glyphosate-resistant common waterhemp, giant ragweed, and kochia were 78, 251, and 17 g a.i. ha^-1, respectively, compared with 144, 489, and 79,349 g a.i. ha^-1, respectively, for 20-cm-tall plants at 21 d after treatment (DAT). Based on visual control estimates at ED₉₀ level, glyphosate-resistant common waterhemp was the most sensitive at both growth stages; kochia was sensitive at 10-cm height, while giant ragweed was the least sensitive at both growth stages. Irrespective of weed species or growth stage, the ED₉₀ values calculated on the basis of shoot biomass reduction were mostly higher compared with visual control estimates. It is concluded that premix of fluthiacet-methyl plus mesotrione can be potentially used as a POST herbicide in corn for controlling glyphosate-resistant common waterhemp and kochia (≤ 10 cm tall) at the labeled rate (87 g a.i. ha^-1).