The natural stereochemistry at the 13-position of avermectin aglycones, normally .alpha.-oriented or below the plane of the molecule, is inverted or epimerized into the .beta.-position. The procedure starts with the avermectin aglycone compounds where the 13.alpha.-hydroxy group is activated to a mesylate leaving group. The resultant 13-.alpha.-mesylate is then displaced with a cesium carboxylate and one equivalent of the respective carboxylic acid, ending with the ester group being converted to the 13-.beta.-hydroxy configuration by transesterification in high yield.