This paper includes a study of the biology of Ascia monuste orseis (Godart, 1818) (Lep., PieHdidae) which was carried out in Campinas and Piracicaba (State of São Paulo, Brasil), during the months of december (1951) and January (1952). References to the eggs and the posture, the behaviour of the female during the oviposition, the larval stages, the chrysalid, the capacity of oviposition of the female and the natural enemies of the species have been made.