Carbonic anhydrase activity in selected leaves was examined in relation to zinc supply to Eucalyptus maculata seedlings and Trifolium subterraneum grown in the glasshouse on a zinc-deficient sand. In both species, carbonic anhydrase activity was closely related to the zinc supply and fell to 0 in severely-deficient plants. Enzyme activity increased linearly as a function of the leaf zinc concentration. In E. maculata plants supplied with adequate zinc, carbonic anhydrase levels were only 3% of those in T. subterraneum. It is concluded that foliar zinc analysis is more sensitive than the carbonic anhydrase assay for determining the zinc status of E. maculata seedlings. Critical zinc concentration for young fully-expanded E. maculata leaves was between 9 and 10 μg g-1 dry matter.