Gill's mealybug, Ferrisia gilli (Gullan) (Hemiptera: Pseudococcidae), is a major pest of pistachio in California. Insecticide treatment is the primary control method and acetamiprid is widely used to control this pest. However, there have been numerous reports of control failures for F. gilli after field applications of recommended insecticides in recent years. The purpose of this study was to develop a method for routine monitoring of F. gilli susceptibility and quantify current levels of F. gilli susceptibility to acetamiprid. A leaf-dip bioassay method using lima bean leaves was established and baseline susceptibility responses of 5 field populations were determined. Lethal concentrations to kill 50% of population (LC₅₀) for second instar nymphs at 48 h ranged from 0.367 to 2.398 µg(AI)ml^–1 of acetamiprid. Similarly, lethal concentrations to kill 90% of population (LC₉₀) for second instar nymphs at 48 h ranged from 2.887 to 10.752 µg(AI)ml^–1 of acetamiprid.The F. gilli population collected from Hanford area showed up to 6.5-fold significantly decreased mortality to acetamiprid compared to other populations.The resistance identified in this study, although relatively low, indicates that there has been repeated pressure to select for acetamiprid resistance and resistance levels can further magnify if effective management steps are not taken. The baseline susceptibility established in this study can be used to investigate potential cause of recent acetamiprid failures against F. gilli. In the long-term, results of this study will support the development of resistance management strategies by monitoring shifts in the susceptibility of F. gilli populations.