Many experiments have evaluated the effects of decreasing soil water contents on top growth and yield of plants, but few experiments simultaneously have evaluated root growth.An experiment was conducted to determine the response of cotton (Gossypium hirsutum L. ‘Auburn 623b’) roots and tops to decreasing soil water content. Plants were grown m rhizotron compartments with rainfall excluded by metal covers over the soil. Soil profile or irrigation schedule treatments provided different levels of soil and plant water potentials. Soil water content was determined with a neutron probe; rooting density, from measurements of roots on the glass viewing surface of each compartment. Plant water potential was determined with a pressure chamber apparatus, and top growth was evaluated by plant height.At the same time that total root length stopped increasing plant top growth slowed or ceased, and plant water potential near sunrise decreased. Root length ceased to increase when the soil water content of any layer decreased to about 0.06 to 0.07 cm3/cm3, which corresponds to a soil water potential slightly wetter than −1 bar or a hydraulic conductivity of about 2 ✕ 10−4 cm/day. Thus, for conditions of this experiment, cotton root growth was correlated with decreases both in plant water potential and in soil water content.