Observations on the growth and development of Chilo partellus (Swinhoe) larvae under laboratory and field conditions indicate sexual dimorphism as well as a general developmental polymorphism. Larvae of both sexes went through fifth, sixth or seventh instars before pupation. The mean developmental period, larval head capsule width, and the number of instars before pupation as well as pupal weight were higher in the female compared to the male larvae. These observations suggest differences in the physiological state of fifth-instar larvae in a population; some of them might be in the penultimate, while others might be in the ultimate instar.
