The Avermectins are a family of macrocyclic lactones isolated from the soil bacterium Streptomyces avermitilis that possess broad spectrum activity as anthelmintics and are employed throughout the agricultural and animal health industries. Avermectin is the parent molecule from which several important parasiticides have been derived, but currently the Ph. Eur. and USP do not describe an HPLC monograph for analysis of Avermectin. Here, we detail the development and validation of a reversed-phase HPLC method for determination of Avermectin and its related substances in bulk batches of Avermectin. The separation of all peaks of interest was accomplished by a gradient elution on an ACE UltraCore (core shell) 2.5 Super C18 column (150 mm × 4.6 mm, 2.5 µm particle size) maintained at 45 °C. The mobile phase for the gradient is 5 mM NH