Studies defining the mode of action of spinosad are summarized, using the American cockroach as an experimental insect. Spinosad is a naturally occurring mixture of two closely related macrocyclic lactones, known as spinosyns, produced by the actinomycete Saccharopolyspora spinosa. In vivo studies showed that spinosyns caused widespread excitation of neurons in the central nervous system, leading to involuntary muscle contractions and tremors. At a threshold dose, spinosyn A was estimated by radiotracer measurements to reach an internal equivalent aqueous concentration of approximately 20 nM, and this concentration was sufficient to directly excite the isolated cockroach central nervous system. Furthermore, in isolated neurons, the excitation was found to be due to persistent activation of nicotinic acetylcholine receptors and prolongation of acetylcholine responses by a novel mechanism that distinguishes spinosad from all other nicotinic agonists. Under certain conditions, spinosyns also had effects on g-aminobutyric acid receptors, but their contribution to symptoms has not been established. Because of its novel mode of action, spinosad has an excellent resistance management profile; with no known cross-resistance, it can be rotated with all other classes of existing and experimental products. Nevertheless, DowElanco is recommending a strong, pro-active resistance management program for Tracer® in cotton.