1. Two peroxidases, differing in ionic character and substrate specificity, have been isolated from the tropical marine sponge Iotrochota birotulata. 2. Both peroxidases catalyze the oxidation of a number of substrates, and one peroxidase possesses a specificity similar to the terrestrial fungal enzyme chloroperoxidase. 3. Based on inhibition studies utilizing sodium azide, potassium cyanide and 8-hydroxyquinoline, it appears that the peroxidases from I. birotulata are haemoprotein complexes. 4. One peroxidase appears to possess subunit structure, and requires bound divalent metal cations for activity.