The effects of cadmium on the catalase activity and isozyme patterns under light and dark conditions of rice(Oryza sativa L. cv. Dongjin) seedlings were examined. Cadmium treatment resulted in the notable enhancement of $H_2O_2$ contents in the seedling roots and leaves under light and dark conditions. The catalase isozyme patterns in the roots were different from those in the leaves, showing tissue-specific expression of the enzyme. Moreover, the expression patterns of catalase isozymes in the green seedling roots were different from those in the etiolated seedling roots following cadmium treatment. The increase of total catalase activity was about 16 times at 1 mM cadmium and marked inductions of the isozyme CAT1 and CAT2 contributed to this increase in the green seedling roots. On the other hand, in the etiolated seedling roots, total catalase activity was lower than that of control at 0.5 and 1 mM cadmium, even though catalase activity increased about 3 times at 0.1 mM cadmium. The 3 fold increase of total catalase activity was mainly due to the increase of CAT1, CAT3 and CAT4 at 0.1 mM cadmium. However, treatment with higher concentrations of cadmium decreased the activity of CAT2 and CAT4 in the etiolated roots. In the leaves, the catalase existed as three isozymes; one cationic isozyme CATc, one neutral isozyme CATn and one anionic isozyme CAT1 in the control. The isozyme patterns and total activities remained unaffected by cadmium under light and dark conditions in the seedling leaves. Taken together, it seems that cadmium-induced changes of catalase might be regulated by light in the roots, but not in the leaves.