The present invention relates to milbemycin derivatives of the formula I ##STR1## wherein X is hydrogen or .beta.-halogen, R is methyl, ethyl, isopropyl or sec-butyl and Az is a 5 membered heterocyclic aromatic ring which contains 2-4 nitrogen atoms and is attached in the 1-position and which is unsubstituted or substituted by one or two C.sub.1 -C.sub.6 alkyl groups. These compounds, and the acid addition salts and metal complexes thereof, are effective pesticides for controlling endo-and ectoparasites, especially for controlling neamatodes which are parasites of animals. The may be obtained by appropriate esterification in 5-position of milbemycin derivatives. The selective .beta.-halogenation of 14,15-epoxymilbemycin derivatives can be effected via the intermediate .DELTA..sup.13,14 -15-hydroxymilbemycins with appropriate halogenating agents.