Seedlings of Phaseolus vulgaris, grown at 22.5 °C in moist vermiculite for 7 days, were transferred to nutrient solution held either at ambient temperature or at 10 °C, or subjected to a root–pruning treatment which removed all lateral roots and part of the main root. The effect of both treatments was to reduce primary leaf growth rate, within 3–6 h in the one case and 12 h in the other. The lower growth rate was maintained for the whole of the expansion period so that the final size of primary leaves on treated plants was only 35–40% of the controls. The smaller leaf size was caused by a reduction in average cell size judged from epidermal cell surface area and anatomical studies. Cell number per leaf was not affected by treatment. Root cooling caused an ephemeral initial fall in bulk leaf water potential, estimated by pressure chamber, but water stress was not thought to be the cause of the inhibition of leaf growth.