Efficacy of Compound 1080 LP Collars was studied under pen and field conditions. Coyotes poisoned themselves by attacking collared sheep and biting the collars. In 54 pen tests where 1 or 2 captive coyotes had opportunity to attack 1 collared lamb, 41 lambs were attacked and 26 collars were punctured. Of 25 different coyotes offered lambs with collars containing 5 or 10 mg sodium fluoroacetate (FAC)/ml, 23 coyotes attacked and 21 died after collars were punctured in their first (n = 17), second (n = 3), or fifth (n = 1) test. For 11 captive coyotes that punctured rubber collars, the average time to death was 217 min (range 115 to 436 min). Collars were placed on approximately 3 percent of the sheep on 4 Idaho and 7 Montana sheep ranches. Coyotes attacked 67 uncollared and 68 collared sheep, punctured 32 collars, and may have punctured 2 other collars that were not found. Documented rates of collar puncture were 48% for all attacks on collared sheep and 64% for neck-throat attacks. Eight collars were punctured on fences, thorns or brush. All coyotes that punctured collars probably died, but only 3 dead coyotes were found. Adverse impacts on humans, domestic animals, and nontarget wildlife were not seen. The LP Collar is a safe, effective, and selective technique for removing coyotes that attack sheep.