Arrhenius plots of translocation velocity as a function of petiole temperature show a marked increase in temperature dependence below 10 C in bean (a chilling-sensitive species) but not in sugar beet (chilling-resistant). The increased temperature dependence below 10 C was not observed for cytoplasmic streaming or oxygen uptake in bean. Bean petioles were served to release pressure in order to determine whether sieve tubes are obstructed in cold-treated petioles. The resulting pressure release caused serious displacement of the crystalline protein bodies in the sieve tubes of petioles at 25 C, but in those locally cooled to 0 C for 30 minutes little displacement occurred, indicating obstruction in the latter. An ultrastructural study of sieve tubes in tissue frozen rapidly in situ and dehydrated by freeze substitution revealed that treatment at 0 C for 30 minutes caused structural alteration and displacement of the cytoplasmic material lining the sieve tube wall resulting in occlusion of sieve plates. The sieve plates of the control petioles at 25 C were generally clear of obstructions. The results indicate that inhibition of translocation by chilling in chilling-sensitive plants results from physical blockage of sieve plates rather than from direct inhibition of a metabolic process which drives translocation.