Roots play an important role in plant survival during periods of drought. A 3-yr study was performed to investigate the effect of drought stress on daily root growth rates of soybean [Glycine max (L.) Merr. 'Braxton') plants grown in a Marvyn loamy sand (fineloamy, siliceous, thermic Plinthic Paleudult) at the Auburn rhizotron. During each growing season, plants received either rainfall (NI) or rainfall and supplemental irrigation (IR) controlled by tensiometers installed at a 0.4-m depth. Root growth was estimated by measuring the length of new roots visible at the glass viewing surface at regular intervals throughout the growing season. Bins were excavated at the end of the 1983 growing season to estimate total root biomass. Roots of both IR and NI plants grew in the upper soil regions (above 0.4 m) during early vegetative growth (V1-V4) and then gradually penetrated into deeper layers, at a rate of 0.5 to 1 m/(m 2 day). A large increase in root growth (up to 6 m/(m 2 day)) was observed when drought stress occurred during later stages of vegetative growth or early reproductive development (R1-R2). Root growth was less affected by drought (<1 m/(m 2 day)) after plants had reached the pod development stage (R4) and finally ceased during seed fill (RS). The IR plants mainly produced roots in soil layers above 0.6 m, while roots of NI plants penetrated deeper soil layers (>0.6 m), especially during drought periods. After rainfall, root growth rates decreased in both treatments. At the end of the season, NI plants had significantly larger root systems (total root length) than IR plants, but total root mass (tap roots and secondary roots) between the two treatments was similar.Additio111ll index words: Glycine max (L.) Merr., Growth analysis, Root length, Root density, Root mass, Root distribution, Soil water potential.