Thirty White Leghorn hens were dusted with P32-labeled Co-Ral® (O, O-diethyl O-(3-chloro-4-methylumbelliferone) phosphorothioate or Bayer 21/199) at 50 mg./kg. Twenty hens were dusted once, while 10 hens received two applications at weekly intervals. Some intact Co-Ral remained on the feathers and skin for 28 days after treatment. The liver, kidney, and bone contained more radioactivity than other internal tissues. By 3 days after treatment, acetonitrile-soluble residues in internal tissues were negligible. The eggs contained minute quantities of acetonitrilc-soluble residues at 5 to 7 days after treatment of the hens, but these residues were not characterized as Co-Ral or its oxygen analog. Radioactive materials excreted in the feces consisted of residual Co-Ral, the oxygen analog, O, O-diethyl phosphoric and O, O-diethyl phosphorothioic acids. Phospholipids, ribose nucleic acid, desoxyribose nucleic acid and acid-soluble phosphorus compounds were isolated from the liver and feces.