Irradiation of V i c i a f a b a root tips with 2.3‐Mhz ultrasound at 10 W/cm2 causes a small but measurable heating of the tips and an immediate reduction in root growth rate. Roots exposed to comparable temperature increments without ultrasound show no reduction in growth, thus leading to the conclusion that the mechanism responsible for the biological effects of ultrasound on roots is nonthermal. From the time course of ultrasonic heating of the roots it appears that the thermal conductivity of the root material is comparable to that of water, and the acoustic absorption coefficient of the root tissue is higher than that of mammalian tissues. Subject Classification: 80.40; 35.75.