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Abstract: The capability of Phascolosoma arcuatum to detoxify sulfide in anaerobic conditions was examined. Sulfane sulfur, which underwent cold cyanolysis, was the major excretory end product of sulfide detoxification during anoxia. Thiosulfate was not excreted into the external medium. Instead, it was absorbed by P. arcuatum and its absorption was stimulated by the presence of sodium sulfide (Na2S) in the incubation medium. The effective formation and excretion of sulfane sulfur by P.␣arcuatum required the presence of both Na2S and sodium thiosulfate (Na2S2O3). Results obtained indicate that rhodanese might be involved in sulfide detoxification in this sipunculid. Rhodanese could act as a catalyst in the transfer of sulfur atoms from thiosulfate to HS−. The body wall and the introvert were the main sites of sulfide detoxification. However, it is unlikely that epibiotic bacteria associated with the outside surface of the worm were involved in the detoxification process. A time-course study on the contents of thiosulfate and sulfane sulfur in the body wall of P. arcuatum incubated anaerobically in the presence of Na2S + Na2S2O3 verified that thiosulfate absorbed was utilized to detoxify sulfide to sulfane sulfur.