An enclosed 4-acre marsh at the southwestern edge of Lake Erie was treated by helicopter with 3. 9 millicuries of chlorine-36 ring-labeled DDT at a rate of 0. 2 lb technical DDT per acre. Radiolabeled DDT residues were traced until 15 months after the application. Whole body residues were of major concern, although tissues were assayed from vertebrates too large to assay entire. DDT residues were detected in sago pondweed Potamogeton pectinatus), duckweed (Lemna minor), bladderwort Utricularia vulgaris), crayfish (Orconectes immunis), tadpoles Rana pipiens), and carp (Cyprinus carpio) when 1st assayed 4 hrs. after the application. The alga Cladophora averaged 96 ppm DDT, or 3125 times the environmental level, 3 days after the application. A northern water snake (Natrix sipedon) accumulated 36 ppm 13 months after the application. The maximum level in carp was 19 ppm in the soft palate, also after 13 months. Plants and most invertebrates accumulated their highest residues during the first week; vertebrates required longer periods. Algae, the pondweeds (Potamogeton spp.), duckweed, crayfish, and small carp should make good indicator species of environmental DDT residues.