Technical Abstract: Physiological responses to Fe-deficiency were characterized in the roots and leaves of avocado plants. These responses included sharply higher catecholase activity, but slightly lower peroxidase activity in leaves and roots of avocado seedlings grown in minus-Fe nutrient solutions, and in Fe-deficient leaves of field trees. Fe- deficiency in avocado seedlings resulted in much higher rates of MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) reduction on the root surfaces. Similarly, the initial rates of FeHEDTA reduction were greater for roots of Fe-deficient avocado seedlings. In contrast, root respiration rates were relatively unaffected by Fe-deficiency. Large differences in the Mn and Zn concentrations in the leaves of the Fe-deficient and Fe-sufficient avocado seedlings suggested that the physiological responses that occurred in the roots during Fe-deficiency influenced the uptake of other metal micronutrients.