Abstract: "This is the preliminary report of a radiological-ecological survey of the invertebrate fauna that inhabit the Columbia River within the confines of Hanford Works and downstream to the site of McNary Dam. The survey was carried out during the period of October, 1948 through February, 1950. Materials and methods are discussed and the results of extensive radioassays, qualitative and quantitative biological determinations, and hydrographic studies are given and analyzed. Twelve figures and twenty-three tables are included. All aquatic invertebrates were found to be concentrating radioactivity from the river water. A maximum activity density of 4.4-10-(-3) μc/g wet weight was found in the larvae of midges (Hydrobaeninae) collected near Hanford during September, 1949. Radioactive wastes were not found to be causing any apparent deleterious effects to the natural invertebrate fauna. Existing population variations are caused by biological and hydrographic conditions."