Baseline toxicity levels to foliarly applied spirotetramat were established for 19 field populations of whiteflies, Bemisia tabaci (Gennadius) B biotype (= Bemisia argentifolii Bellows & Perring) (Hemiptera: Aleyrodidae) from Arizona and California in 2008 and 2009. The susceptibility data were determined against the second instar of B. tabaci field collections before the registration and widespread use of spirotetramat in California. Three strains of whitefly, resistant to either bifenthrin, imidacloprid, or pyriproxyfen, were also tested to determine the potential for cross-resistance to spirotetramat. No significant geographic variation in susceptibility to spirotetramat was observed among regions within Arizona. The LC₅₀ values for the Arizona populations spanned a 14-fold range between populations during the 2 yr sampling tests including a low LC₅₀ of 0.91 (µg [AI] ml^-1) and a high LC₅₀ of 13.47 (µ [AI] ml^-1), while the LC₉₀ values showed a seven-fold range. The field populations from California exhibited limited variation in susceptibility to spirotetramat in general (1.02–7.02 µg [AI] ml^-1) with one exception (27.98 µg [AI] ml^-1). Variation in susceptibility among the resistant strains was about eight-fold at the LC₅₀ level with the PYR-strain, showing the highest susceptibility to spirotetramat at 3.79 (µg [AI] ml^-1). In addition, comparisons of relative susceptibilities among three older immature instars of two field populations showed no significant differences. These results establish a regional baseline that can serve as a reference for future monitoring and management of B. tabaci resistance to spirotetramat.