Using the catechol Tiron as an O2-. scavenger, we showed that sea sponges (Sycon sp.) produce superoxide radicals in sea water at a high rate without any stimuli added. The rate of O2-. outflow from sponges to their water surroundings reaches a value of 0.5 nmol/min per sponge at pH 6.5. The generation of O2-. was inhibited by Cu,Zn-superoxide dismutase, and restored by the addition of KCN. We also confirmed the abiotic production of O2-. in sea water, detected earlier with a different method by Petasne and Zika [Nature 325 (1987) 516-518].