Sixteen American bison, Bison bison, were artificially infected with 10⁵ infective stage larvae of Ostertagia ostertagi on 21 April 1993. At 42 days post-infection eight bison were treated with 0.5% ivermectin pour-on (500 μg/kg bodyweight) and eight treated with the carrier only. Bison were necropsied 17 and 18 days post-treatment (21 and 22 June 1993, respectively). Mean (±SE) of 5,413 (±1,716) adults and 565 (±305) immature O. ostertagi were recovered at necropsy from bison treated with the carrier. No O. ostertagi were detected in bison treated with ivermectin pour-on. Based on the levels of the ivermectin marker metabolite in liver and adipose tissue 18 days post-treatment, the established bovine withdrawal time of 48 days appears adequate to insure that violative residues do not occur.