γ-Cyhalothrin was proposed as an agent for management of lesser mealworm, Alphitobius diaperinus (Panzer) (Coleoptera: Tenebrionidae), in Australian broiler houses. From 2007 to 2009, baseline susceptibility of 20 Australian broiler farm beetle populations plus an insecticide-susceptible laboratory population was determined for γ-cyhalothrin by using topical application. In addition, repeat testing and regression analyses of specific beetle populations to γ-cyhalothrin showed that topical application was a very reliable and repeatable testing method. The 21 populations were tested with a cyfiuthrin discriminating concentration (based on LC_99.9, 0.0007% [AI]) to identify possible cross-resistance. Across all populations, there was a significant linear relationship between the γ-cyhalothrin LC₅₀ value and mortality induced by the cyfiuthrin LC_99.9. Full cyfiuthrin baseline studies of seven populations indicated that γ-cyhalothrin was twice as toxic as cyfiuthrin, even against susceptible beetles and resistance ratios at the cyfiuthrin LC₅₀ and LC_99.9 varied considerably, with maxima of 56.6 and 83.6 respectively. Corresponding ratios for γ-cyhalothrin for the same populations were 8.6 (LC₅₀) and 7.9 (LC_99.9). There were no significant correlations between beetle weights and γ-cyhalothrin LC₅₀ or LC_99.9 values. A discriminating concentration of 0.005% (AI) γ-cyhalothrin was chosen to detect any future changes in susceptibility. Results of this study suggest that cyfiuthrin can confer cross-resistance to γ-cyhalothrin in A. diaperinus, but the magnitude of this resistance is unpredictable. Thus, widespread and frequent cyfiuthrin use in broiler houses in eastern Australia, which has selected for cyfiuthrin resistance, also has resulted in reduced susceptibility to γ-cyhalothrin. Due to its higher relative toxicity, γ-cyhalothrin is still potentially useful for management of lesser mealworm, but due to cross-resistance issues, adoption of γ-cyhalothrin for broiler house use will require a cautious and judicious approach.