Technical Abstract: Soybean [Glycine max (L.) Merr.] seed yields are substantially reduced by drought stress most years on the southeastern Coastal Plain. A better understanding of how drought stress affects soybean seed-yield determination would aid in the development of improved cultivars for this region and better production systems aimed at ameliorating the effects of drought stress. The objectives of this field study were to examine the effects of drought stress on soybean branch vegetative growth, branch seed yield, and total seed yield and to determine how drought stress affects the distribution of seed yield and yield components between the main stem and branch fractions. Soybean was grown in 1998 and 1999 on an Eunola loamy sand with treatments of irrigation and no deep tillage, deep tillage but no irrigation, and no deep tillage or irrigation. Total seed yield and the percentage of total seed yield on branches were highest with irrigation, followed by the deep tilled/no irrigation and the no deep tillage/no irrigation treatments. Irrigation had no effect on mainstem seed yield, but increased branch seed yield by an average of 107%, compared to the no deep tillage/no irrigation treatment. On a unit area basis, branch seed number was highly correlated with branch seed yield and total seed yield over both years and all levels of soil water treatment. Also, branch seed number was closely related to branch dry weight at harvest maturity, final branch length, and final branch number. Most branch growth occurred between initial flowering and the beginning of seed fill. These data indicate that drought stress occurring between initial flowering and early seed fill decreases total seed yield primarily by reducing branch vegetative growth, which reduces branch seed number and branch seed yield.