An experiment was conducted in 2005 and 2006 in southwestern Ontario to examine the efficacy of imazethapyr application timing on the control of long-spined sandbur in soybean (Glycine max L. Merr.). Imazethapyr (100 g a.i. ha-1) was applied early and delayed preemergence (PRE), and postemergence (POST) at the spike to five-leaf stage of the weed. Soybean was not injured by any treatment. Imazethapyr applied PRE did not control long-spined sandbur; POST treatments resulted in up to 72, 66, 78, 91, 78, and 71% weed control when applied at the spike, one-, two-, three-, four-, and five-leaf stage, respectively, and crop yield increased 44, 28, 39, 24, 38, and 20%, respectively. Therefore, imazethapyr should be applied at the three-leaf stage of the weed to optimize control and crop yield. Key words: Weed control, soybean, preemergence herbicide, postemergence herbicide