Mesotrione [2-(4-methylsulfonyl)-2-nitrobenzoyl)-1,3 cyclohenanedione], marketed by Syngenta as Callisto, is a selective herbicide belonging to the triketone family  for use in corn crops.  This herbicide has gained widespread use as a replacement in Europe where atrazine has been banned.  The objective of this study was to determine the impact of repeated herbicide applications on the degradation of the herbicide in three different Texas soils and study the effect of atrazine on mesotrione degradation in a soil microcosm study.  The initial soil microbial respiration experiment was carried out to measure soil carbon mineralization hourly for 33 days kept at an average temperature of 24ºC to observe the influence of mesotrione on carbon mineralization utilizing Orelia fine sandy loam soils.  Soils with a treatment mixture of mesotrione and atrazine, with an application rate of 0.225 kg ha ^-1 for mesotrione and 2.2 kg ha ^-1 for atrazine, produced the greatest amount of CO₂ over the course of the experiment.  When mesotrione was applied to the soil samples alone, the amount of CO₂ produced was less than the mixture treatment.  The total amount of carbon mineralized in the soil samples treated with mesotrione only was similar to the amount mineralized in untreated soil.  Subsequent experiments will determine the effects of repeated herbicide applications.  Evaluating the behavior of mesotrione and the effects on the activity and  composition of soil microbial communities will not only contribute to the limited information concerning the chemical, but it will also serve as a preparation step in understanding the behavior of the chemical where it could potentially be used if atrazine were banned in the United States.