The insect growth regulator tebufenozide (MIMIC 2LV) was tested to examine its impact on the Douglas-fir tussock moth, Orgyia pseudotsugata (McDunnough) (Lepidoptera: Lymantriidae). Laboratory tests gave an estimated concentration of 1.26 ppm of the compound to achieve 50% population mortality (LC₅₀) for second-instar O. pseudotsugata. At the highest concentration tested, tebufenozide resulted in significant larval mortality within 7 d with an estimated time to 50% population mortality (LT₅₀) of 6.3 d. A field comparison of tebufenozide with diflubenzuron (Dimilin 4L) and Bacillus thurengiensis var. kurstaki (Btk, FORAY 48B) was also conducted. There was no significant difference in larval mortality within field plots that were treated with diflubenzuron (42.4%) or Btk (44.8%). Larval mortality in the tebufenozide-treated plots (56.8%) was also similar to the mortality in the diflubenzuron and Btk treatments. All three treatments resulted in more larval mortality than that measured in untreated control plots (11.2%). Both tebufenozide and diflubenzuron treatments resulted in significantly more mortality (55.0% and 40.0%, respectively) to larvae-fed treated foliage 3 wk after application than was measured for larvae-fed foliage from untreated trees (11.0%). There was no significant difference among the treatments in the percentage of host trees in the overstory that sustained >25% defoliation, and all three treatments resulted in less defoliation than was measured in the control plots. There was no significant difference among the treatments in the percentage of host trees in the understory that sustained >25% defoliation.