InEphestia cautella, Corcyra cephalonica andTribolium castaneum, multiple and viable supernumerary larval moults are brought about by treatment of the last larval instar with juvenoids. The normal last larval instar responds to such treatment throughout its life but the number of extra moults is age and dose-related. Supernumerary instars show progressive increase in weight, size and head width. Dyars Law of growth is not strictly applicable. Until a certain stage, a single treatment or removal from juvenoid mixed medium is followed by 2 or 3 further moults and the insects metamorphose normally. Administration of juvenoids with food is more effective than topical treatment.