Root tips from the marsh grass Spartina alterniflora, collected from areas of high and low pore-water sulfide, exhibited a substantial capacity to catalyze sulfide oxidation, as determined by closed-chamber respirometry. A large proportion of this catalysis was apparently nonenzymatic and was higher in roots of plants from the high-sulfide versus the low-sulfide site. Activity exhibiting characteristics of enzymatic sulfide oxidation was significantly higher in plants from the low-sulfide site. Results from elemental analysis of root tissue were consistent with the theory that metals play a role in nonenzymatic catalysis. These results indicate that estuarine plants may detoxify environmental sulfide via sulfide oxidation.