The design and synthesis of a series of avermectin affinity probes used in the identification and purification of the avermectin binding proteins is described. These modified avermectins fall into two design classes: ligands to covalently modify specific avermectin binding proteins [an 125I-labeled aryl azide photoprobe (15) and a tritiated aziridine analog (6)] and ligands for affinity chromatography applications [three biotinylated compounds (10, 12, and 13) and one resin-bound derivative (9)]. The binding affinities of these compounds for the Caenorhabditis elegans avermectin binding protein is presented as well as their biological activities against C. elegans and Artemia salina.