The egg, larvae, pupae, and imago of Microdulia mirabilis (Rothschild), an insect that defoliates native Nothofagus obliqua Mirb. (Oerst.), are described. M. mirabilis distributes between 35 degrees and 47 degrees S in Chile and Neuquen, Argentina. Aspects of the life cycle associated with the duration of different stages of development are given. Immature stages, the imago, and the genitalia are illustrated.