Photodegradation of 4"-(epimethylamino)-4"-deoxyavermectin Bu as thin films on glass, using artificial light, resulted in the formation of multiple photodegradates. Six primary degradates and two secondary degradates were identified. The primary degradates formed were 8,9-Z-4"-(epimethylamino)-4"deoxyavermectin Bu (a geometric isomer), delta 2,3-4"-(epimethylamino)-4"-deoxyavermectin Bu (a positional isomer), avermectin Bu monosaccharide, 4"-epiamino-4"-deoxyavermectin Bu, 4"-N-formyl-4"-deoxyavermectin Bu, and 4"-iV-methyl-N-formyl-4,,-deoxyavermectin Bu. The secondary degradates formed, both geometric isomers of two primary degradates, were 8,9-Z-4"-epiamino-4"deoxyavermectin Bu and 8,9-Z-4"-JV-formyl-4"-deoxyavermectin Bu. The substitution of the epi-iVmethylamino group for the hydroxyl group at the 4" position of avermectin Bu profoundly affected the formation of photodegradates in thin films reported in earlier studies.