Fresh mid- and hind-gut contents of young, hatchery-reared Atlantic salmon degradedp,p′-DDT top,p′-TDE in vitro. Most of the ability to degrade DDT, attributable to intestinal microflora, was eliminated by streptomycin and by heat-sterilization of the gut contents. Since TDE is less toxic than DDT, the activity of intestinal microorganisms may increase the chances of survival of fish that ingest DDT-contaminated prey.