Detailed studies have been carried out on a uraniferous lignite from the Mendenhall strip mine, Harding County, S. Dak. By means of heavy-liquid separations, a mineral-free concentrate of the lignite was obtained that contained 13.8 percent ash and 0.31 percent uranium in the ash. The minerals (gypsum 69 percent, jarosite 10 percent, quartz 2 percent, kaolinite and clay minerals 19 percent, and calcite trace) contain only 7 percent of the uranium in the original coal, indicating an association of the uranium with the organic components of the lignite.Batch extractions show that 88.5 percent of the uranium can be extracted from the lignite by two consecutive treatments with boiling 1 N hydrochloric acid. Continuous extraction with hot 6 N hydrochloric acid removes 98.6 percent of the uranium.Columns of coal were treated with water, 1 N hydrochloric acid, 6 N hydrochloric acid, and a solution of lanthanum nitrate. The experiment with lanthanum nitrate indicated that only 1.2 percent of the uranium in the coal is held by ion exchange. The elutriation experiments showed that the uranium is held in the coal as an organo-uranium compound or complex that is soluble at a pH of less than 2.18.A geochemical mechanism by which the uranium may have been introduced into and retained by the lignite is discussed.