This study reports the isoflavone contents of 210 soybean cultivars grown in South Dakota and explores possible relations between isoflavone contents and agronomic characteristics. Total isoflavone contents (normalized) ranged from 1161 to 2743 μg/g. A number of agronomic characteristics were documented for each variety including maturity group, hilum color, disease resistance, seed weight, yield, maturity (in days), and plant height. Varieties in maturity group I had significantly higher total isoflavones when compared to maturity group 0. Hilum color was related to differences in genistin, daidzein, and genistein content. No differences in isoflavone content were observed based on disease resistance profiles. Genistein content was found to be negatively correlated with yield, days of maturity, and plant height. Weak but significant correlations also existed between these agronomic characteristics and other isoflavones.