Increases of greenhouse gases, especially atmospheric carbon dioxide concentration, are expected to cause global mean temperatures to increase.  We need to know the impacts of global warming on crop yields. As experimental growing season temperatures are increased above the optimum temperature for a given crop, seed yields decline and drop to zero at about 10 °C above the optimum. The objectives of this research were to determine the impact of elevated temperatures (+4.5 °C) above local baseline temperatures (Gainesville, Florida) on the seed yields of soybean cultivars in maturity group (MG) I to IX.  Soybean was grown in field soil in replicated 2-m rows in four temperature-gradient greenhouses at baseline and elevated (baseline + 4.5 °C) temperatures, with two each temperature-gradient greenhouses at ambient (370 ppm) and at elevated (700 ppm) carbon dioxide concentrations. Twenty-four cultivars of MG V to IX soybean were grown in 2004 and 22 cultivars of MG I to VI were grown in 2005.  Growth stages were recorded as each cultivar developed.  As they matured, each cultivar was harvested for measurements of above-ground biomass [stems and (pods + seeds)]. Seed yields and harvest index [seed dry weight/(stem + podwall + seed) dry weight] were calculated.  Overall, harvest index was lower at elevated temperatures above baseline (+4.5 °C) and also in elevated carbon dioxide. There was a range in the sensitivity of cultivars to high temperature, which indicates presence of genetic variability that could be used in future development of high temperature tolerant soybean cultivars.