The Bureau of Mines investigated the feasibility of utilizing low- grade phosphatic shale as a feed for phosphoric acid production. Present practice in the phosphate mining operations of southeastern Idaho is to remove and either stockpile or waste low-grade phosphatic shales as the high-grade phosphate beds are selectively mined. Acid digestion studies were performed on uncalcined shale and on shale calcined to remove free carbon to determine phosphoric acid quality, yield, and amenability to filtration. The untreated shale, containing 21.4 pct p2o5, was digested with sulfuric acid in a single-tank reactor to yield phosphoric acid and gypsum residue. Although 96 pct p2o5 recovery was attained in digesting uncalcined shale, only the calcined product gave satisfactory filtration rates. Calcined shale yielded phosphoric acid recovery of about 93 pct of the contained p2o5. It was higher in impurities than commercially prepared acid derived from high-grade western phosphate rock. An acid filtration rate of 745 lb p2o5/ft2/day was attained. This value is lower than the rate of 900 lb p2o5/ft2 that is generally considered to be acceptable in industrial production of phosphoric acid from high-grade phosphate rock.