The yellowheaded fireworm,Acleris minuta(Robinson), is a multivoltine pest of rosaceous and ericaceous plants. Larval development of both sexes may proceed through five, six, or seven stadia. In field populations, the five-instar mode is more prevalent during first, second, and third generations; frequency of the six- and seven-instar modes increases during the fourth generation. Reduced growth rates increase frequencies of supernumerary molts. A discriminant function based on head-capsule size and sex classified 70% of the laboratory population under the proper mode of development. This discriminant function was used to classify field populations into the proper mode of development.