In dark-grown leaves of Zea mays, the catalase (H2O2:H2O2 oxidoreductase; EC 1.11.1.6; CAT) isozyme CAT-2 is absent. With continuous white light, CAT-2 protein levels increase (due to de novo synthesis) and plateau after 24 hr. When total poly(A)+ RNA (mRNA), polysomes, or isolated polysomal mRNA from light- and dark-treated leaves was translated in vitro, CAT-2 was detected only among the light-treated leaf products. A Cat2 clone was used to probe blots of total mRNA and polysomal mRNA from light- and dark-treated leaves. Cat2 mRNA was found in approximately equal quantities in both. Cat2 mRNA was equally distributed in identical high molecular weight fractions in polysomes from light- and dark-treated leaves and, therefore, was probably not sequestered in ribonucleoprotein particles in dark-grown leaves. The control of Cat2 expression appears to involve a unique form of translational inhibition in dark-grown leaves, preventing translation of the isolated mRNA and preventing polypeptide elongation. These results may have important implication in studies of translational control in other systems.