Abamectin is a novel macrocyclic lactone natural product produced by fermentation of Streptomyces avermitilis. Abamectin (Zephyr^® 0.15 EC) is highly toxic to field isolates of the twospotted spider mite, Tetranychus urticae Koch, collected from cotton in the San Joaguin Valley of California. LC50 values for contact toxicity of abamectin for the field isolates fell within the range of 0.014 - 0.077 ppm for residual toxicity. However, these field isolates exhibited varying levels of resistance to the standard cotton acaricides, dicofol and propargite.