Field trials were conducted in 2002 and 2003 at seven sites to determine the optimum rates of mesotrione and atrazine applied PRE for minimal crop injury and control of common lambsquarters, velvetleaf, Pennsylvania smartweed, common ragweed, giant ragweed, ivyleaf morningglory, and common cocklebur. All rates of each herbicide resulted in greater than 95% control of triazine-susceptible common lambsquarters. Mesotrione at 105 g ai/ha resulted in greater than 90% control of triazine-resistant common lambsquarters, velvetleaf, and Pennsylvania smartweed. Control of common ragweed was 90% or greater from mesotrione at 158 g/ha in combination with atrazine at 280 g/ha or greater. In addition, mesotrione at 210 g/ha combined with any rate of atrazine provided at least 92% control of common ragweed. Combinations of mesotrione and atrazine only suppressed, and did not effectively control, giant ragweed, common cocklebur, and ivyleaf morningglory.