Abstract Life in the sulfide system requires the ability to withstand the toxic effects of long-term sulfide stress. The adaptations of sulfide-system interstitial metazoans to sulfide stress have been investigated using 35S-sulfide. One possible adaptation, the total exclusion of sulfide at the body wall, could not be demonstrated. Sulfide-system turbellarians and gastrotrichs, however, consistently incorporated less 35S-sulfide during a 2-h incubation period than their surface-dwelling counterparts. Analysis of autoradiographs of the sulfide-system acoels Myopea crassula, Solenofilomorpha funilis, and Pseudohaplogonaria sp., and the surface-dwelling proseriate Archiloa wilsoni revealed major differences in the concentration of 35S label in the body wall relative to the internal tissues. Timed uptake and loss experiments plus data from autoradiography showed that the label in the body wall was not permanently bound but, instead, was easily lost. The data indicate that a sulfide detoxification system exists in the body wall which uses an as yet unknown scavenging system to trap much of the sulfide that enters, detoxifies it, and releases the end-product to the medium.